Fix RX descriptor writeback threshold to 1 as default

MAC write-back threshold should be set to 1 as default, this threshld will be refered to control the number of receive descriptors in the on-chip buffer that are ready to be written back to host memory. for EtherCAT, it need poll receive descriptors on time, if this value is bigger than 1, the ethercat stack will occur datagram timeout.

Signed-off-by: Zhang, Wei E wei.e.zhang@intel.com