Fix modpost failure when rx handlers are not inlined.

When CONFIG_MAC80211_MESH=n and CONFIG_MAC80211_NOINLINE=y,
gcc doesn't optimize out a call to ieee80211_rx_h_mesh_fwding,
even if the previous comparison is always false in this case.
This leads to the following errors during modpost:

ERROR: "mpp_path_lookup" [net/mac80211/mac80211.ko] undefined!
ERROR: "mpp_path_add" [net/mac80211/mac80211.ko] undefined!

Fix by removing the possibility of uninlining
ieee80211_rx_h_mesh_fwding rx handler.

Signed-off-by: Davide Pesavento <davidepesa@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>