regmap: Make regmap_noinc_read() return -ENOTSUPP if map->read isn't set

Before adding support to define bulk read/write callbacks in regmap_config
by the commit d77e74561368 ("regmap: Add bulk read/write callbacks into
regmap_config"), the regmap_noinc_read() function returned an errno early
a map->bus->read callback wasn't set.

But that commit dropped the check and now a call to _regmap_raw_read() is
attempted even when bulk read operations are not supported. That function
checks for map->read anyways but there's no point to continue if the read
can't succeed.

Also is a fragile assumption to make so is better to make it fail earlier.

Fixes: d77e74561368 ("regmap: Add bulk read/write callbacks into regmap_config")
Signed-off-by: Javier Martinez Canillas <[e-mail]>
Reviewed-by: Marek Vasut <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/base/regmap/regmap.c

···
 	size_t read_len;
 	int ret;
 
+	if (!map->read)
+		return -ENOTSUPP;
+
 	if (val_len % map->format.val_bytes)
 		return -EINVAL;
 	if (!IS_ALIGNED(reg, map->reg_stride))