mpc83xx: Fix the UEC driver bug of QE

The patch prevents the GCC tool chain from striping useful code for
optimization. It will make UEC ethernet driver workable, Otherwise the
UEC will fail in tx when you are using gcc4.x. but the driver can work
when using gcc3.4.3.

CHANGELOG

*Prevent the GCC from striping code for optimization, Otherwise the UEC
will tx failed when you are using gcc4.x.

Signed-off-by: Dave Liu <daveliu@freescale.com>