MAINTAINERS: add entry for ethtool

MAINTAINERS: add entry for ethtool

Michal did an amazing job converting ethtool to Netlink, but never
added an entry to MAINTAINERS for himself. Create a formal entry
so that we can delegate (portions) of this code to folks.

Over the last 3 years majority of the reviews have been done by
Andrew and I. I suppose Michal didn't want to be on the receiving
end of the flood of patches.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250204215729.168992-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>