drivers/usb/gadget/udc: Assign boolean values to a bool variable

Fix the following coccicheck warnings:

./drivers/usb/gadget/udc/udc-xilinx.c:1957:2-18: WARNING:
Assignment of 0/1 to bool variable.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Jiapeng Zhong <abaci-bugfix@linux.alibaba.com>
Link: https://lore.kernel.org/r/1610615002-66235-1-git-send-email-abaci-bugfix@linux.alibaba.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>