iwlwifi: bump required firmware API version for 3160/7260

As the firmware API has changed significantly and we don't
have support code for the old APIs, bump the version to be
able to release the version 7 API firmware. Unfortunately
this means that the driver in 3.9 and 3.10 can't work, but
that's still better than crashing the device/driver there.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>