mac80211: avoid unnecessary beacon deref on CSA counter update

The beacon struct is already available in many contexts that
are also already in an RCU read-locked section. Avoid that by
using the existing beacon struct pointer directly.

Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
[rewrite subject/add commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>