rt2x00: Move rt2800_txdone and rt2800_txdone_entry_check to rt2800usb.

These two functions are only used by rt2800usb so they don't have to be
in rt2800lib.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>