ARM: shmobile: r8a7778: fix Ether device name

While recasting commit 52421914 (ARM: shmobile:
R8A7778: add Ether support), I made a typo in the platform device's name: used
underscore instead of hyphen.

However, there's now patch merged to net-next.git renaming the platform device
from "sh-eth" to "r8a777x-ehter", so it makes the most sense to change the name
straight to that one.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>