[bwn] ensure the fallback rate stays in the same PHY mode.

Falling back from 6MB OFDM to 5MB CCK (a) may not work well in the
11bg PHYs, (b) won't work at all if you're 11g only, and (c) plainly
won't work for the 11a PHY.

So, don't do that!

Tested:

• BCM4312 802.11b/g Wireless, STA mode WLAN (chipid 0x4312 rev 15) PHY (analog 6 type 5 rev 1) RADIO (manuf 0x17f ver 0x2062 rev 2)