net: phy: at803x: generalize cdt fault length function

Generalize cable test fault length function since they all base on the
same magic values (already reverse engineered to understand the meaning
of it) to have consistenct values on every PHY.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>