[media] media: i2c/adp1653: fix check of devm_gpiod_get() error code

The devm_gpiod_get() function returns either a valid pointer to
struct gpio_desc or ERR_PTR() error value, check for NULL is bogus.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>