mfd: ene-kb3930: Fix a potential NULL pointer dereference

mfd: ene-kb3930: Fix a potential NULL pointer dereference

The off_gpios could be NULL. Add missing check in the kb3930_probe().
This is similar to the issue fixed in commit b1ba8bcb2d1f
("backlight: hx8357: Fix potential NULL pointer dereference").

This was detected by our static analysis tool.

Cc: stable@vger.kernel.org
Fixes: ede6b2d1dfc0 ("mfd: ene-kb3930: Add driver for ENE KB3930 Embedded Controller")
Suggested-by: Lee Jones <lee@kernel.org>
Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
Link: https://lore.kernel.org/r/20250224233736.1919739-1-chenyuan0y@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>