iwlagn: allow up to uCode API 6 for 6000 devices

commit b914811524fbe9e91fe50845f5d7bd4316b8a6ee upstream.

Since the uCode hasn't been released (yet?),
warn only if using older than API 4, but load
anything up to API 6.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>