wifi: cfg80211: use IEEE80211_MAX_MESH_ID_LEN appropriately

Even if that's the same as IEEE80211_MAX_SSID_LEN, we really
should just use IEEE80211_MAX_MESH_ID_LEN for mesh, rather
than having the BUILD_BUG_ON()s.

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