net: phy: modify assignment to OR for dev_flags in phy_attach_direct

Modify the assignment to OR when dealing with phydev->dev_flags in
phy_attach_direct function, and this is to make sure dev_flags set in
driver's probe callback won't be lost.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
CC: Heiner Kallweit <hkallweit1@gmail.com>
CC: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Tao Ren <taoren@fb.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>