gpio: bcm281xx: Use "unsigned gpio" consistently throughout the code

This patch removes some inconsistencies caused by the use of "int gpio"
in some parts of the code and "unsigned gpio" in others.

Signed-off-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>