gianfar: Consider dts property endianess on handling

Use of_property_read*() to get arch endian consistent
property values. Do some refactoring in the process.

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