V4L/DVB: dvb: siano: free spinlock before schedule()

Calling schedule() holding spinlock with disables irqs is improper. As
spinlock protects list coredev->buffers, it can be unlocked untill wakeup.
This bug was introduced in a9349315.

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>