lan78xx: Return 0 when lan78xx_suspend() has no error.

lan78xx_suspend() may return non-zero from lan78xx_write_reg() in some scenario.
Fix to return 0 when lan78xx_suspend() has no error.

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>