b43: don't unconditionally fall back to CCK if the rate is 6MB OFDM.

Check the current PHY operating mode (gmode) to see if we should
fall back from 6MB OFDM to 11MB CCK.  For 5GHz operation this isn't
allowed.

Note, the fallback lookup is only done for RTS rates; normal fallback
rates are done via mac80211 and aren't affected by this change.

Signed-off-by: Adrian Chadd <adrian@freebsd.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>