iwlegacy: fix dma mappings and skbs leak

Fix possible dma mappings and skbs introduced by commit
470058e0 "iwlwifi: avoid Tx queue
memory allocation in interface down".

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>