net: pcs: xpcs: add specific vendor supoprt for Wangxun 10Gb NICs

Since Wangxun 10Gb NICs require some special configuration on the IP of
Synopsys Designware XPCS, introduce dev_flag for different vendors. Read
OUI from device identifier registers, to detect Wangxun devices.

And xpcs_soft_reset() is skipped to avoid the reset of device identifier
registers.

Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>