gpio: max3191x: Use bitmap_free() to free bitmap

kfree() and bitmap_free() are the same. But using the later is more
consistent when freeing memory allocated with bitmap_alloc().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>