rt2x00: Fix header_length in rt2x00lib_txdone

Put the assignment of header_length after pull out extra tx headroom

Signed-off-by: RA-Jay Hung <jay_hung@ralinktech.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>