iwlwifi: module parameter to enable/disable bt co-exist

iwlwifi: module parameter to enable/disable bt co-exist

Adding "bt_coex_active" module parameter for iwlcore to enable/disable
BT coexist; if bt_coex_active is true (default), uCode will do kill/defer
every time the priority line is asserted (BT is sending signals on the
priority line in the PCIx). By disable the bt_coex_active, uCode will
ignore the BT activity and perform the normal operation.

Users might experience transmit issue on some platform due to this
WiFi/BT co-exist problem. The possible symptoms are: NetworkManager and
other similar programs can scan and find all the available APs, but will
timeout and unable to associate with any of the APs; no out-going frames
can be found with wireless sniffer tools.

On those platforms, WiFi communication can be restored by set "bt_coex_active"
module parameter to "false"

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>