iwlwifi: be more verbose about needed firmware

If the supported firmware versions are not found, we currently only
print "no suitable firmware found".  This is not very informative for
the user trying to find the correct version to use.  Improve this by
printing the exact firmware name(s) the driver supports and pointing
to the git repository where they can be found.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>