iwlwifi: incorporate firmware filename into version

We have many different firmware images with the same version,
and it's sometimes cumbersome to figure out which image was
really used, especially as the marketing strings that we do
print out can be the same for (slightly) different hardware
using different firmware images.

Incorporate the firmware filename into the fw_version so it's
printed out all the time. Unfortunately, this will make the
string be longer than the 32 characters for ethtool, but we
almost never really use ethtool, so strip the "iwlwifi-"
prefix (if not overridden), and the remaining data that may
then be stripped at the end is not usually useful anyway.

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