net: fix auto-loading of Marvell DSA driver

Auto-loading of the Marvell DSA driver has stopped working with recent
kernels.  This seems to be due to the change of binding for DSA devices,
moving them from the platform bus to the MDIO bus.

In order for module auto-loading to work, we need to provide a MODALIAS
string in the uevent file for the device.  However, the device core does
not automatically provide this, and needs each bus_type to implement a
uevent method to generate these strings.  The MDIO bus does not provide
such a method, so no MODALIAS string is provided:

.# cat /sys/bus/mdio_bus/devices/f1072004.mdio-mii\:04/uevent
DRIVER=mv88e6085
OF_NAME=switch
OF_FULLNAME=/soc/internal-regs/mdio@72004/switch@4
OF_COMPATIBLE_0=marvell,mv88e6085
OF_COMPATIBLE_N=1

In the case of OF-based devices, the solution is easy -
of_device_uevent_modalias() does the work for us.  After this is done,
the uevent file looks like this:

.# cat /sys/bus/mdio_bus/devices/f1072004.mdio-mii\:04/uevent
DRIVER=mv88e6085
OF_NAME=switch
OF_FULLNAME=/soc/internal-regs/mdio@72004/switch@4
OF_COMPATIBLE_0=marvell,mv88e6085
OF_COMPATIBLE_N=1
MODALIAS=of:NswitchT<NULL>Cmarvell,mv88e6085

which results in auto-loading of the Marvell DSA driver on Clearfog
platforms.

Fixes: c0405563

 ("ARM: dts: armada-388-clearfog: Utilize new DSA binding")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>