mac80211: avoid crashing on invalid band info

Frequent crashes have been observed on MT7916 based platforms. While the
root of these crashes are currently unknown, they happen when decoding
rate information of connected STAs in AP mode. The rate-information is
associated with a band which is not available on the PHY.

Check for this condition in order to avoid crashing the whole system.
This patch should be removed once the roout cause has been found and
fixed.

Closes: #2980
Link: https://github.com/freifunk-gluon/gluon/issues/2980



Signed-off-by: David Bauer <mail@david-bauer.net>