iwlagn: allow up to uCode API 6 for 6000 devices

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: John W. Linville <linville@tuxdriver.com>