ath5k: update PCU opmode whenever a new interface is added

Previously, we would store the operating mode at interface up time,
but only update the PCU registers when the next reset happened.
The result is that if beacon configuration (ops->bss_info_changed)
happens before ops->config, we will program the wrong things into
the timer registers.  Consequently, beacons won't work in AP mode
until after a reset (channel change, scan etc.).

This is fragile anyway so just program the opmode as soon as
mac80211 gives it to us.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>