gianfar: ensure ECNTRL[R100] is cleared on link state change

When changing the link between 100Mbps and 1Gbps in SGMII mode it was
found out that the link would stop working.  The issue is that ECNTRL[R100]
needs to be cleared when in 1Gbps mode.  Older reference manuals didn't
require the explicitly clearing but has since been found it that it is
needed.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>