ANDROID: GKI: ethtool.h: add Android ABI padding to a structure

Try to mitigate potential future driver core api changes by adding a
padding to struct ethtool_ops.

Based on a change made to the RHEL/CENTOS 8 kernel.

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: If9f449cb74e81d048a1c41b33c6b7c5b564ecaf3