ath5k: simplify passed params to ath5k_hw_set_associd()

We have access to common->curbssid and common->curaid so just
use those. Note that common->curaid is always 0 so this keeps
our current behaviour of always using 0 for now. Once we fix
storing the association ID passed by mac80211 this will
require no changes here.

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