dmaengine: qcom: bam_dma: Fix completed descriptors count

One space is left unused in circular FIFO to differentiate
'full' and 'empty' cases. So take that in to account while
counting for the descriptors completed.

Fixes the issue reported here,
	https://lkml.org/lkml/2019/6/18/669

Cc: [email protected]
Reported-by: Srinivas Kandagatla <[email protected]>
Signed-off-by: Sricharan R <[email protected]>
Tested-by: Srinivas Kandagatla <[email protected]>
Signed-off-by: Vinod Koul <[email protected]>

Author: Sricharan R

drivers/dma/qcom/bam_dma.c

@@ -799,6 +799,9 @@ static u32 process_channel_irqs(struct bam_device *bdev)
 		/* Number of bytes available to read */
 		avail = CIRC_CNT(offset, bchan->head, MAX_DESCRIPTORS + 1);
 
+		if (offset < bchan->head)
+			avail--;
+
 		list_for_each_entry_safe(async_desc, tmp,
 					 &bchan->desc_list, desc_node) {
 			/* Not enough data to read */