gpio: palmas: fix a possible NULL dereference

of_match_device could return NULL, and so cause a NULL pointer
dereference later.

Reported-by: coverity (CID 1130700)
Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>