gpio: loongson1: fix bgpio usage

When no flags are given, the native endianness is used to access
the MMIO registers, and the pin2mask() call can simply be
converted to a BIT() call, as per the default pin2mask()
implementation in gpio-mmio.c.

Cc: Kelvin Cheung <keguang.zhang@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>