iwlagn: provide data after WARN_ON

From time to time, we hit a WARN_ON in iwl_mac_remove_interface.
This basically means that we got out of sync with mac80211: the vif
we hold differs from the vif 80211 passes as parameter. Try to get
some data that will help to debug this.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>