[MAC80211]: use switch statement in tx code

The transmit code needs to set the addresses depending on the
interface type, a likely() for AP/VLAN is quite wrong since
most people will be using STA; convert to a switch statement
to make it look nicer.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>