net: phy: marvell: Configure TDR pulse based on measurement length

When performing a TDR measurement for a short distance, the pulse
width should be low, to help differentiate between the outgoing pulse
and any reflection. For longer distances, the pulse should be wider,
to help with attenuation.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>