media: pvrusb2: fix warning in pvr2_i2c_core_done

syzbot has reported the following warning in pvr2_i2c_done:

	sysfs group 'power' not found for kobject '1-0043'

When the device is disconnected (pvr_hdw_disconnect), the i2c adapter is
not unregistered along with the USB and v4l2 teardown. As part of the USB
device disconnect, the sysfs files of the subdevices are also deleted.
So, by the time pvr_i2c_core_done is called by pvr_context_destroy, the
sysfs files have been deleted.

To fix this, unregister the i2c adapter too in pvr_hdw_disconnect. Make
the device deregistration code shared by calling pvr_hdw_disconnect from
pvr2_hdw_destroy.

Reported-by:  <syzbot+e74a998ca8f1df9cc332@syzkaller.appspotmail.com>
Tested-by:  <syzbot+e74a998ca8f1df9cc332@syzkaller.appspotmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>