dt-bindings: net: renesas,ether: Update Sergei's email address

Update Sergei's email address, as per commit 534a8bf0
("MAINTAINERS: switch to my private email for Renesas Ethernet
drivers").

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