mac80211: don't call drv_set_default_unicast_key() for VLANs

Since drivers know nothing about AP_VLAN interfaces, trying to
call drv_set_default_unicast_key() just results in a warning
and no call to the driver. Avoid the warning by not calling the
driver for this on AP_VLAN interfaces.

This means that drivers that somehow need this call for AP mode
will fail to work properly in the presence of VLAN interfaces,
but the current drivers don't seem to use it, and mac80211 will
select and indicate the key - so drivers should be OK now.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>