iwlagn: bump firmware API for some devices

We're working on improvements for the firmware
for some devices, and need to bump the API for
those since they won't be backward compatible
completely (the earlier patch reserving queue
10 for P2P).

Bump the API version to 6 for those devices
but don't warn users of version 5 yet.

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>