mwl8k: properly report rate on received 40MHz packets

On 8366, bit 6 in the rx descriptor rate field indicates whether the
packet was received on a 20MHz or 40MHz channel, and is not part of
the MCS index.  Handle this properly, which then prevents hitting the
WARN_ON and being dropped in ieee80211_rx().

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>