[media] cx18: Move spinlock and vb_type initialisation into stream_init

The initialisation of vb_type in serialized_open was preventing
REQBUFS from working reliably. Remove it, and move the spinlock into
stream_init for good measure - it's only used when we have a stream
that supports videobuf anyway.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>