gpio: 104-idio-16: Define maximum valid register address offset

Attempting to load the 104-idio-16 module fails during regmap
initialization with a return error -EINVAL. This is a result of the
regmap cache failing initialization. Set the idio_16_regmap_config
max_register member to fix this failure.

Fixes: 2c210c9a34a3 ("gpio: 104-idio-16: Migrate to the regmap API")
Reported-by: Mark Cave-Ayland <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]
Suggested-by: Mark Cave-Ayland <[e-mail]>
Cc: [e-mail]
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: William Breathitt Gray <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpio-104-idio-16.c

···
 	.reg_stride = 1,
 	.val_bits = 8,
 	.io_port = true,
+	.max_register = 0x5,
 	.wr_table = &idio_16_wr_table,
 	.rd_table = &idio_16_rd_table,
 	.volatile_table = &idio_16_rd_table,