gpio: loongson: Use BIT() macros

This switches the Loongson driver over to using the bitops BIT()
macros and drops some local variables and make the code easier
to read (in my opinion).

Cc: Keguang Zhang <keguang.zhang@gmail.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: linux-mips@linux-mips.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>