Don't call ath_init() inside the lock.

Yes, this means that sc_invalid is slightly racy, but there are other
issues here which need fixing.

This fixes a source of eventual LORs - ath_init() grabs ATH_LOCK to do
work and releases it before it calls ieee80211_start_all().
ieee80211_start_all() will grab the net80211 comlock to iterate over
the VAPs.

TODO:

• .. I should just migrate the ieee80211_start_all() work to a deferred task so it can be done later; it doesn't have to be immediately done.

Tested:

• AR5416, STA mode