net: sh-eth: Fix misaligned cache operation warning

When we using network on board using sh-eth, it prints a lot of
"CACHE: Misaligned operation at range" messages.
This commit fixes this problem.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>