iwlagn: name change for bt_ch_announce module parameter

Change the module parameter name to bt_ch_inhibition from
bt_ch_announce to better describe the functionality

In order to allow Bluetooth to activate a smart AFH mechanism
and to maximize its available bandwidth the WiFi will request
BT Core to inhibit its activity in channels that interfere
with WiFi activity (and vice versa) if bt_ch_inhibition is enabled

Set module parameter "bt_ch_inhibition=0" will disable the channel
inhibition function

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>