Allow the user to configure where on the cable the TDR data should be
retrieved, in terms of first and last sample, and the step between
samples. Also add the ability to ask for TDR data for just one pair.

If this configuration is not provided, it defaults to 1-150m at 1m
intervals for all pairs.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>

v3:
Move the TDR configuration into a structure
Add a range check on step
Use NL_SET_ERR_MSG_ATTR() when appropriate
Move TDR configuration into a nest
Document attributes in the request
Signed-off-by: David S. Miller <davem@davemloft.net>