net: dsa: dsa_loop: Make dynamic debugging helpful

Remove redundant debug prints from phy_read/write since we can trace those
calls through trace events. Enhance dynamic debug prints to print arguments
which helps figuring how what is going on at the driver level with higher level
configuration interfaces.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>