extcon: max77843: Replace irqchip mask_invert with unmask_base

Remove use of the deprecated mask_invert flag. Inverted mask
registers (where a '1' bit enables an IRQ) can be described more
directly as an unmask register.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>