ath9k_hw: skip WEP aggregation enable code for AR9003

The AR9002 hardware code enables aggregation for WEP but
mac80211 doesn't enable aggregation with WEP, and the AR9003
code family does not need this so skip it for now for AR9003
but leave the code and annotate we should eventually consider
how to remove this in consideration for the HAL unification
goals.

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