extcon: gpio: Request reasonable interrupts

The only thing that makes sense is to request a falling edge interrupt
if the line is active low and a rising edge interrupt if the line is
active high, so just do that and get rid of the assignment from
platform data. The GPIO descriptor knows if the line is active high
or low.

Also make irq a local variable in probe(), it's not used anywhere else.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[cw00.choi: Fix build error of data->irq_flags]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>