wlan: Let driver holds WLAN serializer for ieee80211_ifattach

wlan: Let driver holds WLAN serializer for ieee80211_ifattach

Looks like almost all building drivers (except, ndis(4) and wpi(4))
hold WLAN serializer for ieee80211_ifattach(), so we release the
WLAN serializer in ieee80211_ifattach() before the if_attach().

This reverts 4109d03b12096f353e4036119a0d6357fa1ec67e and
part of a583ece6b440118aa26f3eb162a1df7f2821f923.

Reported-by: marino@