b43: remove rate index warning

I frequently run into this warning, and added some
debugging to see why, and got this:

b43 bad rx: 00000000: 2f d2 e2 63 cf a7 14 04 28 18 c8 5f 88 4a a2 00
                      bogus junk       | plcp            | fctl| dur

b43 bad rx: 00000010: 00 11 24 91 07 4d 00 06 25 ff 8f 78 00 06 25 ff
                      my MAC address   | BSSID           | AP MAC
b43 bad rx: 00000020: 8f 76 20 74 00 00 42 07 00 20 00 00 00 00 aa aa
                           | seq | QoS | CCMP IV               | data
                      ...

As you can see, there are 6 bogus bytes (sometimes only five) and then
the frame. I don't know why, and I don't see how to recover, so let's
just drop these frames.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>