sfp: fix non-detection of PHY

The detection of a PHY changed in commit e98a3aab ("mdio_bus: don't
return NULL from mdiobus_scan()") which now causes sfp to print an
error message.  Update for this change.

Fixes: 73970055 ("sfp: add SFP module support")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>