of: mdio: preserve phy dev_flags in of_phy_connect()

Replace assignment "=" with OR "|=" for "phy->dev_flags" so "dev_flags"
configured in phy probe() function can be preserved.

The idea is similar to commit e7312efb ("net: phy: modify assignment
to OR for dev_flags in phy_attach_direct").

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>