net: phy: fix null dereference in phy_attach_direct

Commit bc66fa87 ("net: phy: Add link between phy dev and mac dev")
introduced a link between net devices and phy devices. It fails to check
whether dev is NULL, leading to a NULL dereference error.

Fixes: bc66fa87

 ("net: phy: Add link between phy dev and mac dev")
Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>