mac80211: mesh: clean up rx_bcn_presp API

We currently pass the entire elements to the rx_bcn_presp()
method, but only need mesh_config. Additionally, we use the
length of the elements to calculate back the entire frame's
length, but that's confusing - just pass the length of the
frame instead.

Link: https://lore.kernel.org/r/20210920154009.a18ed3d2da6c.I1824b773a0fbae4453e1433c184678ca14e8df45@changeid


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