virtio_net: reject unsupported coalescing params

Set ethtool_ops->supported_coalesce_params to let
the core reject unsupported coalescing parameters.

This driver correctly rejects all unsupported parameters.
As a side effect of these changes the error code for
unsupported params changes from EINVAL to EOPNOTSUPP.

v2: correctly handle rx-frames (and adjust the commit msg)
v3: adjust commit message for new error code and member name

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>