gpio: zynq: Remove call to platform_get_irq

Remove the call to  platform_get_irq use the cached
one instead.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>