rt2x00: Fix rate detection for invalid signals

It has been observed on rt2500pci hardware that some
frames received with signal 0x0C do not have the OFDM
flag set.

Signals can have 2 meanings:
 1) The PLCP value
 2) The bitrate * 10

For rt2500pci (1) is for frames received with a OFDM rate,
and (2) is for frames received with a CCK rate.
But 0x0C is a invalid bitrate value but is a valid PLCP
value for 54Mbs (obvious OFDM rate).
This means that it is possible that the hardware does not
set the OFDM bit correctly under all circumstances.
This results in rt2x00 failing to detect the rate and
mac80211 triggering a WARN_ON() and dropping the frame.

To bypass this, print a warning when such a frame is received,
and reset the rate to the lowest supported rate for the current band.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>