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.