gpio: zynq: Check return value of pm_runtime_get_sync

Return value of "pm_runtime_get_sync" API was neither captured nor checked.
Fixed it by capturing the return value and then checking for any warning.

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