net: cdc_ncm: use sysfs for rx/tx aggregation tuning

Attach a driver specific sysfs group to the netdev, and use it
for the rx/tx aggregation variables.

The datagram aggregation defined by the CDC NCM specification is
specific to this device class (including CDC MBIM). Using the
ethtool interrupt coalesce API as an interface to the aggregation
parameters redefined that API in a driver specific and confusing
way.  A sysfs group
 - makes it clear that this is a driver specific userspace API, and
 - allows us to export the real values instead of some translated
   version, and
 - lets us include more aggregation variables which were impossible
   to force into the ethtool API.

Additionally, using sysfs allows tuning the driver on space
constrained hosts where userspace tools like ethtool are undesired.

Suggested-by: Peter Stuge <peter@stuge.se>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>