tests: Fix wlan.mesh.config.cap workaround for test_wpas_mesh_max_peering

tests: Fix wlan.mesh.config.cap workaround for test_wpas_mesh_max_peering

The wlan.mesh.config doesn't have to be the last element of beacon. Things
like VHT or HE oper/cap are usually follow the mesh configuration element.

The workaround must first get the position of a correct reference value in
wlan.mesh.config (ps_protocol) and then calculate the correct
wlan.mesh.config.cap offset based on that.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Fixes: 2cbaf0de223b ("tests: Work around tshark bug in wpas_mesh_max_peering")
Signed-off-by: Sven Eckelmann <sven@narfation.org>