um: reject unsupported coalescing params

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

This driver did not previously reject unsupported parameters.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>