iwlwifi: mvm: stop using MIMO in case BT doesn't allow it

Switch to using the new btcoex decision api regarding MIMO and stop
accessing the internal btcoex structs.
In case MIMO should be disabled it would detect this upon the next
Tx and force a search. The search will switch to SISO on a antenna A
which isn't used by BT.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>