gpio: sta2x11: Use BIT() macro

This removes the custom implementation of the BIT() macro
and inlines all calls to the helper.

Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>