net: dsa: better error reporting

Add additional error reporting to the generic DSA code, so it's easier
to debug when things go wrong.  This was useful when initially bringing
up 88e6176 on a new board.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>