Even though a driver or mac80211 shouldn't produce a
legacy bitrate if sband->bitrates doesn't exist, don't
crash if that is the case either.

This fixes a kernel panic if station dump is run before
last_rate can be updated with a data frame when
sband->bitrates is missing (eg. in S1G bands).
Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20201005164522.18069-1-thomas@adapt-ip.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>