media: i2c: mt9p031: Check return value of devm_gpiod_get_optional() in mt9p031_probe()

media: i2c: mt9p031: Check return value of devm_gpiod_get_optional() in mt9p031_probe()

The devm_gpiod_get_optional() function may return an error pointer
(ERR_PTR) in case of a genuine failure during GPIO acquisition, not just
NULL which indicates the legitimate absence of an optional GPIO.

Add an IS_ERR() check after the function call to catch such errors and
propagate them to the probe function, ensuring the driver fails to load
safely rather than proceeding with an invalid pointer.

Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>