mac80211: mesh: use u16 return type for u16 getter

u16_field_get() is a simple wrapper around get_unaligned_le16(),
and it is being assigned to a u16, so there's no need to
promote to u32 in the middle.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>