net: netcp: Fixes SGMII reset on network interface shutdown

This patch asserts SGMII RTRESET, i.e. resetting the SGMII Tx/Rx
logic,  during network interface shutdown to avoid having the
hardware wedge when shutting down with high incoming traffic rates.
This is cleared (brought out of RTRESET) when the interface is
brought back up.

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>