ieee802154: Remove 802.15.4/6LoWPAN checks for interface MTU.

In the past, 802.15.4 interfaces and 6LoWPAN interfaces used the
same dev->type (ARPHRD_IEEE802154), and 802.15.4 interfaces were
distinguished from 6LoWPAN interfaces by their differing dev->mtu.

6LoWPAN interfaces have their own ARPHRD type now, so there is no
longer any need to check dev->mtu to distinguish 802.15.4 devices
from 6LoWPAN devices.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>