ath9k_htc: avoid scheduling while atomic on sta_rc_update

mac80211 ->sta_rc_update() callback must be atomic. Since we have to
take mutex and do other operations that can sleep when sending fimrware
commands to device, the only option to satisfy atomicity requirement of
->sta_rc_update(), that I can see, is introduce work_struct and defer
uploading new rates to that work.

Tested-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>