ath5k: warn and correct rate for unknown hw rate indexes

ath5k sets up a mapping table from the hardware rate index to
the rate index used by mac80211; however, we have seen some
received frames with incorrect rate indexes.  Such frames
normally get dropped with a warning in __ieee80211_rx(),
but it doesn't include enough information to track down the
error.

This patch adds a warning to hw_to_driver_rix for any lookups
that result in a rate index of -1, then returns a valid rate so
the frame can be processed.

Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>