gpio: twl6040: Use bitops

It's nice to use BIT() macros rather than open coding the same.
It's good practice as sometimes people use BIT(31) and forget
that the constant must be cast unsigned long.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>