Add kerneldoc for vendor request functions in rt2x00usb.
Add asynchroneous vendor request function in rt2x00usb.

With the availability of the asynchroneuous vendor request
we can now enable LED class support for rt2500usb and rt73usb.
Since LED handling is not important, it doesn't really matter
if a register call fails (This solution is better then no
LED class support at all).
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>