iwlagn: stop the device before freeing it

When we remove the module, we free all the tx and rx resources.
Before doing that, we'd better stop the tx / rx activity. Calling
iwl_trans_stop_device in iwl_remove helps also to remove a few API
functions:
 * rx_free:		happens in iwl_trans_free
 * tx_free:		happens in iwl_trans_free
 * disable_sync_irq:	happens in iwl_trans_stop_device

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>