iwlwifi: virtualize op_mode's free skb

This handler allows the transport layer to free an skb from the
op_mode. This can happen when the driver is stopped while Tx
packets are pending in the transport layer.

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