gpio: zynq: Fix missing checking return value

LOG:
drivers/gpio/gpio-zynq.c: In function 'xgpiops_probe':
drivers/gpio/gpio-zynq.c:673:17: warning: ignoring return
value of 'gpiochip_remove', declared with attribute
warn_unused_result [-Wunused-result]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>