gpio: gpio-xilinx: Fix integer overflow

Current implementation is not able to configure more than 32 pins
due to incorrect data type. So type casting with unsigned long
to avoid it.

Fixes: 02b3f84d

 ("xilinx: Switch to use bitmap APIs")
Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>