cfg80211: scan: use element finding functions in easy cases

There are a few easy cases where we only check for NULL or
have just simple use of the result, this can be done with
the element finding functions instead.

Link: https://lore.kernel.org/r/20210930131130.f27c8a7ec264.Iadb03c4307e9216e080ce513e8ad4048cd020b25@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>