Do not init SATA when disabled on 8536DS.

SGMII and SATA share the serdes on MPC8536 CPU, When SATA disabled and the
driver still try to access the SATA registers, the cpu will hangup.
This patch try to fix this by reading the serdes status before the SATA
initialize.

Signed-off-by: Jason Jin <Jason.jin@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>