staging: ti dspbridge: avoid errors if stream id is zero

As 'zero' can be a perfectly good id, it can be picked up as
a NULL from userspace, avoid issues in API and user apps if stream
handle is zero.

Signed-off-by: Ernesto Ramos <ernesto@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>