MAINTAINERS: switch to my private email for Renesas Ethernet drivers

I no longer work for Cogent Embedded (but my old email still works :-)),
and still would like to continue looking after the Renesas Ethernet drivers
and bindings.  Let's switch to my private email.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>