net: phy: Check against net_device being NULL

In general, we don't want MAC drivers calling phy_attach_direct with the
net_device being NULL. Add checks against this in all the functions
calling it: phy_attach() and phy_connect_direct().

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>