ethtool: consolidate work with ethtool_ops

No need to check if ethtool_ops == NULL since it can't be.
Use local variable "ops" in functions where it is present
instead of dev->ethtool_ops
Introduce local variable "ops" in functions where dev->ethtool_ops is used
many times.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Reviewed-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>