media: pvrusb2: fix memory leak in pvr_probe

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

media: pvrusb2: fix memory leak in pvr_probe

The error handling code in pvr2_hdw_create forgets to unregister the

v4l2 device. When pvr2_hdw_create returns back to pvr2_context_create,

it calls pvr2_context_destroy to destroy context, but mp->hdw is NULL,

which leads to that pvr2_hdw_destroy directly returns.

Fix this by adding v4l2_device_unregister to decrease the refcount of

usb interface.