Return correct error code if _mv88e6xxx_reg_read returns an error.

Fixes: facd95b2 ("net: dsa: mv88e6xxx: Add Hardware bridging support")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>