iwlwifi: add bt_init_traffic_load as configurable parameter

Adding configurable parameter in .cfg for the initial Bluetooth traffic
load; set it to IWL_BT_COEX_TRAFFIC_LOAD_NONE for now, but can be change
for debugging or other reason.

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