mac80211: don't sanitize invalid rates

I found this bug while poking around with a pure-gn AP.

Commit:
cfg80211/mac80211: Use more generic bitrate mask for rate control

Added some sanity checks to ensure that each tx rate index
is included in the configured mask and it would change any
rate indexes if it wasn't.

But, the current implementation doesn't take into account
that the invalid rate index "-1" has a special meaning
(= no further attempts) and it should not be "changed".

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>