extcon: gpio: Convert to fully use GPIO descriptor

Since we are not getting the GPIO from any platform data and global
GPIO numberspace, we simply get the named "extcon" GPIO directly from
the device. Cut away "active low" since GPIO descriptors already know
if the line is active high or low. Simplify a bit with a
struct device *dev helper variable in probe() and cut the complex
init() function.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>