linux/netdevice.h is included in very many places, touching any
of its dependecies causes large incremental builds.

Drop the linux/ethtool.h include, linux/netdevice.h just needs
a forward declaration of struct ethtool_ops.

Fix all the places which made use of this implicit include.
Acked-by: NJohannes Berg <johannes@sipsolutions.net>
Acked-by: NShannon Nelson <snelson@pensando.io>
Reviewed-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/20201120225052.1427503-1-kuba@kernel.orgSigned-off-by: NJakub Kicinski <kuba@kernel.org>