net: ethtool: Handle missing cable test TDR parameters

A last minute change put the TDR cable test parameters into a nest.
The validation is not sufficient, resulting in an oops if the nest is
missing. Set default values first, then update them if the nest is
provided.

Fixes: f2bc8ad3

 ("net: ethtool: Allow PHY cable test TDR data to configured")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>