iwlwifi: allow selecting only MVM driver

Now that we have two drivers (DVM and MVM) stop selecting
the DVM one (but make it default) and allow enabling only
the MVM driver if so desired. Add a warning for the case
of having neither DVM nor MVM enabled -- that's useless.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>