iwlwifi: mvm: BT Coex - add debugfs hook to set BT Tx priority

In order to debug the firmware, we need to be able to set
the BT priority of WiFi packets. This priority is set based
on the type of the packet (control frames, EAPOL etc...).
For debugging purposes, allow to override this priority by
a debugfs controlled value.
Enable this feature that needs this priority to be able to
test it.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>