mac80211: call set_wmm_default only for valid vifs

mac80211 calls ieee80211_set_wmm_default (which in turn
calls drv_conf_tx()) for every new interface, including
"internal" ones (e.g. monitor interface, which the low-level
driver doesn't know about).

Limit this call only to valid interfaces.

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