ltq-ptm: Support 1508-byte MTU for RFC4638

Tested with VDSL on TP-Link WD8970, I see full 1500-byte PPP data
frames, which end up being 1526 byte Ethernet frames (including
Ethernet+VLAN headers) on the wire.

Fixes: FS#210

Signed-off-by: David Woodhouse <dwmw2@infradead.org>