iwlagn: disable gen2b BT coexistence in IBSS

IBSS doesn't allow for coexistence, so it
should be disabled.

Additionally, disable reacting to the BT
profile notification when in IBSS mode,
it likely won't be sent by the device to
start with though.

Also, in IBSS mode, BT coexistence isn't as fully-featured
and we must use a single antenna only. So instead of
peppering the code with new checks, simply pretend
we are in high BT traffic load, which has the needed
effect of disabling antenna B use.

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