gpio: bcm281xx: Fix parameter name for GPIO_CONTROL macro

The GPIO_CONTROL macro returns the control register offset when given a
GPIO number.

Update the argument name in the macro to reflect that it takes in a
GPIO number and not a bank.

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>