mac80211: refactor non-{sae,dfs} mesh initialization

Refactor mesh initialization into a separate function, do some cleaning
on the way to make the code more readable.
Changes:
 * Move iw mesh setup to new mac80211_setup_mesh()
 * fallback on 'ssid' parameter in case 'mesh_id' isn't set
 * move setting of freq variable to shared code as it is needed for
   both, the wpa_supplicant and the iw based setup.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>