gpio: zynq: Take bank offset into account when reporting a IRQ

When looking up the IRQ the bank offset needs to be taken into account.
Otherwise interrupts for banks other than bank 0 get incorrectly reported as
interrupts for bank 0.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>