mwl8k: handle 5 GHz legacy rate bitmaps in firmware commands

Whenever mac80211 gives us a legacy rate bitmap in the context of the
5 GHz band, we need to remember to shift the bitmap left by 5 positions
before giving it to the firmware, as the firmware follows the bitmap
bit assignment of the 2.4 GHz rate table even if we're on the 5 GHz
band, and the 2.4 GHz rate table includes five non-OFDM rates at the
start that are not valid in the 5 GHz band.

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