gianfar: Replace eieio with wmb for non-PPC archs

Replace PPC specific eieio() with arch independent wmb()
for other architectures, i.e. ARM.
The eieio() macro is not defined on ARM and generates
build error.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>