iwlwifi: mvm: BT Coex - classify packet priority in BT code

This code is really related to BT Coex - move it to the coex
file.
Also - prepare for a FW API change that will happen soon:
Bits 11 and 12 will be allocated for BT priority. Today, we
only have bit 12.

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