rt2x00: Fix TX status reporting for rt2800pci.

After testing, we found that TX_STA_FIFO_MCS is the last MCS value
tried. If the transmission failed, 8 frames have been transmitted. If the
transmission succeed, we can easily compute the number of retry.  This patch fix
the way status is reported to mac80211 rate control. It has 2 bugs :

1. mcs can contain the short preamble flag and it will lead to wrong
computations.

2. minstrel nearly always say that 54 Mbits is the best rate, even if we are
very far from the AP

Signed-off-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: Alban Browaeys <prahal@yahoo.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>