ath5k: Reset key cache on interface up, thus fixing resume

After a s2ram / resume cycle, resetting the key cache does not work
unless it is deferred until after the hardware has been reinitialised by
a call to ath5k_hw_reset(). This fixes a regression introduced by
"ath5k: fix suspend-related oops on rmmod".

Signed-off-by: Elias Oltmanns <eo@nebensachen.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>