mwl8k: properly set receive status rate index on 5 GHz receive

The mwl8k firmware uses indices into the 2.4 GHz band rate table for
the receive descriptor channel field even if the packet was received
on a 5 GHz channel, while mac80211 expects an index into the 5 GHz
band rate table when packets are received on the 5 GHz band, which
presents a mismatch as the 5 GHz band rate table lacks the five
non-OFDM rates that the 2.4 GHz rate table starts with.

To handle this properly, we need to substract 5 from the rate index
field if the packet was received on a 5 GHz channel (and was not
received at an MCS rate).

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