gpio: omap: use BIT() macro instead of shifting bits

Using the BIT() macro instead of shifting bits
makes the code less error prone and also more readable.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>