staging: vt6656: return if pControlURB->hcpriv not NULL

Fixes occasional urb submitted while active.

Even thought the fMP_CONTROL_WRITES/fMP_CONTROL_READS flags
are cleared in the return context urb->hcpriv is not NULL.

check for hcpriv and return STATUS_FAILURE if not NULL.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>