net: phy: at803x: drop specific PHY ID check from cable test functions

Drop specific PHY ID check for cable test functions for at803x. This is
done to make functions more generic. While at it better describe what
the functions does by using more symbolic function names.

PHYs that requires to set additional reg are moved to specific function
calling the more generic one.

cdt_start and cdt_wait_for_completion are changed to take an additional
arg to pass specific values specific to the PHY.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>