phylib: fix premature freeing of struct mii_bus

Commit 46abc021

 ("phylib: give mdio
buses a device tree presence") added a call to device_unregister() in
a situation where the caller did not intend for the device to be
freed yet, but apart from just unregistering the device from the
system, device_unregister() does an additional put_device() that is
intended to free it.

The right function to use in this situation is device_del(), which
unregisters the device from the system like device_unregister() does,
but without dropping the reference count an additional time.

Bug report from Bryan Wu <cooloney@kernel.org>.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Tested-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>