uio_hv_generic: Fix kernel NULL pointer dereference in hv_uio_rescind

In the Linux kernel, the following vulnerability has been resolved:

uio_hv_generic: Fix kernel NULL pointer dereference in hv_uio_rescind

For primary VM Bus channels, primary_channel pointer is always NULL. This

pointer is valid only for the secondary channels. Also, rescind callback

is meant for primary channels only.

Fix NULL pointer dereference by retrieving the device_obj from the parent

for the primary channel.