nl80211: clean up code/policy a bit

Use the policy to validate minimum and exact lengths in
some attributes that weren't previously covered in the
right ways, and remove associated validation code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200805154714.3ba1e233cfa0.I5dc8109b7ab5c3f4ae925f903a30cc9b35753262@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>