extcon: rt8973a: Drop useless mask_invert flag on irqchip

There's no need to set the flag explicitly to false, since that
is the default value from zero initialization.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>