gpio: zynq: Check return value of irq_get_irq_data

In two different instances the return value of "irq_get_irq_data"
API was neither captured nor checked.
Fixed it by capturing the return value and then checking for any error.

Addresses-Coverity: "returned_null"
Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>