usb_relay_device_enumerate: silencing -Wunused-but-set-variable

[Ricardicus]

I was building usb-relay-hid on my machine and ran into this warning:

../../lib/usb_relay_lib.c: In function ‘usb_relay_device_enumerate’:
../../lib/usb_relay_lib.c:382:9: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
int ret;
^~~

There is a return code from usbhidEnumDevices that is not looked at.
Since the returning value pusb_relay_device_info_t is a pointer-type,
I figured if this error appears, NULL is returned.