ath9k_common: move the rate status setting into ath9k_process_rate()

This has no real functional change, this just moves the setting the
the mac80211 rate index into ath9k_process_rate(). This allows us
to eventually make ath9k_process_rate() return a negative value
in case we have detected a specific case rate situation which should
have been ignored.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>