iwlwifi: mvm: BT Coex - prepare towards new API

A new API is coming. This new API is not backward
compatible. So we need to keep the old commands to be able
to work with the former API.
Move all the current code into a new file: coex_legacy.
If a firmware with the new API is detected, we currently
just bail out since the implementation of the new API will
come in future patches.

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