drivers: net: phy: at803x code cleanup on register and unregister driver

Make use of phy_drivers_register/phy_drivers_unregister to register/unregister
multiple phy drivers in a single module.

Cc: Matus Ujhelyi <ujhelyi.m@gmail.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>