V4L/DVB (4849): Add missing spin_unlock to saa6588 decoder driver

Sparse noticed a lock imbalance in read_from_buf(). Further inspection shows
that the lock should not be held when the function exits.
This adds a spin_unlock_irqrestore(), so that every exit path of the
read_from_buf() function is consistent. The unlock was missing on an error
path.
Signed-off-by: NIra W. Snyder <devel@irasnyder.com>
Signed-off-by: NHans J. Koch <koch@hjk-az.de>
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/saa6588.c

@@ -212,8 +212,10 @@ static void read_from_buf(struct saa6588 *s, struct rds_command *a)
 	if (rd_blocks > s->block_count)
 		rd_blocks = s->block_count;
 
-	if (!rd_blocks)
+	if (!rd_blocks) {
+		spin_unlock_irqrestore(&s->lock, flags);
 		return;
+	}
 
 	for (i = 0; i < rd_blocks; i++) {
 		if (block_to_user_buf(s, buf_ptr)) {