iwlwifi: change fw.mvm_fw to fw.type

Instead of explicitly indicating the difference between just
DVM and MVM with an mvm_fw boolean change this to fw.type to
be more extensible and easier to understand.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>