usb: dwc3: Correct datatype of base to unsigned long

Correct type of varibale base to unsigned long as
keeping it as int causes usb failures if MSB of
the base address is set.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>