mac80211: Minor optimization in tx status handling

ieee80211_tx_status iterates over all tx rates the driver reports back
in order to
1) mark tx rates as invalid if the driver cannot have tried that rate
2) find the actually used tx rate for the final retransmission

By leaving the for loop when the first invalid rate index is found we
can move the rates_idx assignment after the loop and therefore save
a few assignments and conditionals.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>