Enhance HID_SendReport method signature

HID_SendReport (USBAPI.h and HID.cpp) does not return any value, although the underlying implementation uses USBSend which returns the number of bytes or -1 if failed. I suggest changing the signature of HID_SendReport to return an int value to enable consumers to detect error conditions.

I'd be willing to do the change in a pull request. Unfortunately I can't discuss that on the mailing list as I do not have a Google account (and I am not willing to get one ;-)).