iwlagn: move iwlagn_stop_device to transport layer

Since iwlagn_stop_device was the only caller to the rx_stop / tx_stop,
these two don't need to be API any more.

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