usb: isp1760: Fix out-of-bounds array access

Running the driver through kasan gives an interesting splat:

BUG: KASAN: global-out-of-bounds in isp1760_register+0x180/0x70c
Read of size 20 at addr f1db2e64 by task swapper/0/1
(...)
isp1760_register from isp1760_plat_probe+0x1d8/0x220
(...)

This happens because the loop reading the regmap fields for the
different ISP1760 variants look like this:

for (i = 0; i < HC_FIELD_MAX; i++) { ... }

Meaning it expects the arrays to be at least HC_FIELD_MAX - 1 long.

However the arrays isp1760_hc_reg_fields[], isp1763_hc_reg_fields[],
isp1763_hc_volatile_ranges[] and isp1763_dc_volatile_ranges[] are
dynamically sized during compilation.

Fix this by putting an empty assignment to the [HC_FIELD_MAX]
and [DC_FIELD_MAX] array member at the end of each array.
This will make the array one member longer than it needs to be,
but avoids the risk of overwriting whatever is inside
[HC_FIELD_MAX - 1] and is simple and intuitive to read. Also
add comments explaining what is going on.

Fixes: 1da9e1c06873 ("usb: isp1760: move to regmap for register access")
Cc: [e-mail]
Cc: Rui Miguel Silva <[e-mail]>
Cc: Dietmar Eggemann <[e-mail]>
Reviewed-by: Rui Miguel Silva <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/isp1760/isp1760-core.c

···
 	[HW_DM_PULLDOWN]	= REG_FIELD(ISP176x_HC_OTG_CTRL, 2, 2),
 	[HW_DP_PULLDOWN]	= REG_FIELD(ISP176x_HC_OTG_CTRL, 1, 1),
 	[HW_DP_PULLUP]		= REG_FIELD(ISP176x_HC_OTG_CTRL, 0, 0),
+	/* Make sure the array is sized properly during compilation */
+	[HC_FIELD_MAX]		= {},
 };
 
 static const struct reg_field isp1763_hc_reg_fields[] = {
···
 	[HW_DM_PULLDOWN_CLEAR]	= REG_FIELD(ISP1763_HC_OTG_CTRL_CLEAR, 2, 2),
 	[HW_DP_PULLDOWN_CLEAR]	= REG_FIELD(ISP1763_HC_OTG_CTRL_CLEAR, 1, 1),
 	[HW_DP_PULLUP_CLEAR]	= REG_FIELD(ISP1763_HC_OTG_CTRL_CLEAR, 0, 0),
+	/* Make sure the array is sized properly during compilation */
+	[HC_FIELD_MAX]		= {},
 };
 
 static const struct regmap_range isp1763_hc_volatile_ranges[] = {
···
 	[DC_CHIP_ID_HIGH]	= REG_FIELD(ISP176x_DC_CHIPID, 16, 31),
 	[DC_CHIP_ID_LOW]	= REG_FIELD(ISP176x_DC_CHIPID, 0, 15),
 	[DC_SCRATCH]		= REG_FIELD(ISP176x_DC_SCRATCH, 0, 15),
+	/* Make sure the array is sized properly during compilation */
+	[DC_FIELD_MAX]		= {},
 };
 
 static const struct regmap_range isp1763_dc_volatile_ranges[] = {
···
 	[DC_CHIP_ID_HIGH]	= REG_FIELD(ISP1763_DC_CHIPID_HIGH, 0, 15),
 	[DC_CHIP_ID_LOW]	= REG_FIELD(ISP1763_DC_CHIPID_LOW, 0, 15),
 	[DC_SCRATCH]		= REG_FIELD(ISP1763_DC_SCRATCH, 0, 15),
+	/* Make sure the array is sized properly during compilation */
+	[DC_FIELD_MAX]		= {},
 };
 
 static const struct regmap_config isp1763_dc_regmap_conf = {