ath5k: fix suspend-related oops on rmmod

Based on a patch by Elias Oltmanns, we call ath5k_init in resume even
if we didn't previously open the device.  Besides starting up the
device unnecessarily, this also causes an oops on rmmod because
mac80211 will not invoke ath5k_stop and softirqs are left running after
the module has been unloaded.  Add a new state bit, ATH_STAT_STARTED,
to indicate that we have been started up.

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Elias Oltmanns <eo@nebensachen.de>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>