nl80211: accept testmode dump with netdev

All nl80211 commands that need only the wiphy
still allow identifying it by giving an interface
index, except, as Kenny pointed out, the testmode
dump support.

Fix this by looking up the wiphy via the ifidx in
this case as well.

Tested-by: Kenny Hsu <kenny.hsu@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>