Staging: intel_sst: cleanup naming a little

&sst_drv_ctx->streams[str_id].lock and &stream->lock are the same.
The mutex_lock() uses &stream->lock so this makes things consistent and
it's nicer to read as well.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>