gpio: pxa: simplify BANK_OFF macro offset calculation

The macro BANK_OFF which calculates the base offset for each GPIO port.
The macro is needlessly complex and unreadable. Simplify the
calculation to a simple math operation.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: linux-gpio@vger.kernel.org
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>