gpio: pch: Optimize pch_gpio_get()

The double negation is costly and can be avoided by shifting the
register value before masking the requested bit.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>