gianfar_ptp: replace get_of_u32 with of_property_read_u32

Replace get_of_u32 with standard helper function of_property_read_u32
since the latter can process cpu endianness.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>