powerpc/fsl_soc: gianfar: don't probe disabled devices

Freescale ships MPC8315E-RDB boards in two variants:

1. With TSEC1 ethernet support and USB UTMI PHY;
2. Without TSEC1 support, but with USB ULPI PHY in addition.

For the second case U-Boot will add status = "disabled"; property
into the TSEC1 node, so Linux should not try to probe it.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>