ieee802154: 6lowpan: use correct ESC value for dispatch

The ESC dispatch value has some history and it originally was 0x7f in rfc4944
(see section-5.1). With the release of rfc6282 this value got part of the
LOWPAN_IPHC range and was no longer available for ESC. Instead 0x40 was used
as replacement (see section-2 in rfc6282).

We have been checking the dispatch byte in an order where IPHC would always be
evaluated before ESC and thus we would never reach the ESC check as the IPHC
range already covers this value.

Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>