patches: remove wpan_dev_hard_header() on kernel < 3.18

wpan_dev_hard_header() accesses ieee802154_ptr which is not available
on older kernel versions, remove this function on such kernel versions.
We do not compile cfg802154 on such kernel versions any way, but the
6lowpan stack uses this header file and we need 6lowpan for bluetooth.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>