gpio: pca953x: fix irq_stat not updated when irq is disabled (irq_mask not set)

When one port's input state get inverted (eg. from low to hight) after
pca953x_irq_setup but before setting irq_mask (by some other driver such as
"gpio-keys"), the next inversion of this port (eg. from hight to low) will not
be triggered any more (because irq_stat is not updated at the first time). Issue
should be fixed after this commit.

Fixes: 89ea8bbe ("gpio: pca953x.c: add interrupt handling capability")
Signed-off-by: Puyou Lu <puyou.lu@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>