mac80211: make ieee80211_new_mesh_header return unsigned

The function returns always non-negative values.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>