nl80211: add handling for BSS color

This patch adds the attributes, policy and parsing code to allow userland
to send the info about the BSS coloring settings to the kernel.

Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20191217141921.8114-1-john@phrozen.org


[johannes: remove the strict policy parsing, that was a misunderstanding]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>