net: dsa: mt7530: make some noise if register read fails

Simply returning the negative error value instead of the read value
doesn't seem like a good idea. Return 0 instead and add WARN_ON_ONCE(1)
so this kind of error will not go unnoticed.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>