net: dsa: b53: Use dev_{err,info} instead of pr_*

This change allows us to see which device the err or info messages are
referring to if we have multiple b53 compatible devices on a board.

As this removes the only pr_*() calls in this file we can drop the
definition of pr_fmt().

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: David S. Miller <davem@davemloft.net>