mac80211: defer master netdev allocation to ieee80211_register_hw

When we want to go multiqueue, we will need to know the number of
queues the hardware has for registering the master netdev. This
number is only available in ieee80211_register_hw() rather than
ieee80211_alloc_hw(), so defer allocation of the master device to
ieee80211_register_hw().

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>