net80211: increase number of spares in struct ieee80211_vap

Turns out MFCing 713db49d06deee90dd358b2e4b9ca05368a5eaf6 does not
leave us with enough spares. Given wireless will likely see more
changes in the near future add more spares.
This is especially necessary given 'struct ieee80211_vap' gets
allocated by drivers.
Bumps size of struct ieee80211_vap to (7 * 512) on 64bit.