net: phy: bcm54140: use genphy_soft_reset()

Set the .soft_reset() op to be sure there will be a reset even if there
is no hardware reset line registered.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>