mac80211: add more missing checks for VHT tx rates

Fixes a crash on attempting to calculate the frame duration for a VHT
packet (which needs to be handled by hw/driver instead).

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>