gpio: gpio-xilinx: Check return value of of_property_read_u32

In two different instances the return value of "of_property_read_u32"
API was neither captured nor checked.
Fixed it by capturing the return value and then checking for any error.

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Addresses-Coverity: "check_return"
Link: https://lore.kernel.org/r/1605201148-4508-9-git-send-email-srinivas.neeli@xilinx.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>