rt2x00: Fix rt2800 retry calculation

The retry count for a frame is calculated by the realized MCS rate
subtracting of the requested MCS rate. However during TX done reporting
we accidently override the requested MCS rate with the realized MCS
rate. This causes the retry count to be reset to 0 always, and feeding
invalid rate information into minstrel_ht.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>