Add missing attribute validation for NL80211_ATTR_OPER_CLASS
to the netlink policy.

Fixes: 1057d35e ("cfg80211: introduce TDLS channel switch commands")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20200303051058.4089398-4-kuba@kernel.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>