ltq-vectoring: add driver

ltq-vectoring: add driver

In order to calculate the required pre-distortion for downstream
vectoring, the vectoring control entity (VCE) at the carrier office
needs error samples from the modem. On Lantiq VR9 modems, error reports
are generated by the firmware, but need to be multiplexed into the data
stream by the driver on the main processor when L2 encapsulation is
selected by the VCE.

This driver provides the necessary callback function, which is called by
the MEI driver after receiving an error report from the firmware.

Originally, it is part of the Lantiq PPA driver, but after a few changes
it also works with the PTM driver used in OpenWrt. The direct call to
ndo_start_xmit needs to be replaced, as the PTM driver relies on locks
from the kernel. Instead dev_queue_xmit is used, which is called from a
work queue, as it is not safe to call from an interrupt handler.

Additional changes include fixes to support recent kernel versions and
a change of the used interface from ptm0 to dsl0.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jan Hoffmann <jan@3e8.eu>
(cherry picked from commit f872b966092ece5c0e2192e0d979a9eb69283f17)