dsa: Fix conversion from host device to mii bus

Commit b4d2394d ("dsa: Replace mii_bus with a generic host device")
replaces mii_bus with a generic host_dev, and introduces
dsa_host_dev_to_mii_bus() to support conversion from host_dev to mii_bus.
However, in some cases it uses to_mii_bus to perform that conversion.
Since host_dev is not the phy bus device but typically a platform device,
this fails and results in a crash with the affected drivers.

BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<ffffffff81781d35>] __mutex_lock_slowpath+0x75/0x100
PGD 406783067 PUD 406784067 PMD 0
Oops: 0002 [#1] SMP
...
Call Trace:
[<ffffffff810a538b>] ? pick_next_task_fair+0x61b/0x880
[<ffffffff81781de3>] mutex_lock+0x23/0x37
[<ffffffff81533244>] mdiobus_read+0x34/0x60
[<ffffffff8153b95a>] __mv88e6xxx_reg_read+0x8a/0xa0
[<ffffffff8153b9bc>] mv88e6xxx_reg_read+0x4c/0xa0

Fixes: b4d2394d ("dsa: Replace mii_bus with a generic host device")
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>