iwlwifi: mvm: support cloned tx skbs

Since tx skbs are not modified by the driver, let mac80211 know the
driver can handle cloned skbs.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>