iwlagn: separate firmware version warning

We sometimes need to support new firmware API for
a while before we can publish them since testing
them fully takes a long time. We could keep all
the new code private, but that causes plenty of
problems and sometimes we can give a pre-release
version of firmware to people who need to test.
However, when we just bump the API version, the
driver will warn everybody that their firmware is
outdated, when in fact it isn't. (Currently our
case for this doesn't really change the API but
bumping the API version is necessary because the
firmware isn't fully backward compatible)

In order to handle this in the future, add a new
"api_ok" version; only below this will the driver
warn that the uCode is too old.

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>