rtl8169: fix cache coherency problem

Fix the problem that cannot access actual data when CPU data cache enabled.

Signed-off-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Tested-by: Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>
Acked-by: Ben Warren <biggerbadderben@gmail.com>