media: ipu3-imgu: Remove dead code for NULL check

Since ipu3_css_buf_dequeue() never returns NULL, remove the
dead code to fix static checker warning:

drivers/staging/media/ipu3/ipu3.c:493 imgu_isr_threaded()
warn: 'b' is an error pointer or valid
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
[Bug report: https://lore.kernel.org/linux-media/20190104122856.GA1169@kadam/]
Signed-off-by: Yong Zhi <yong.zhi@intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/staging/media/ipu3/ipu3.c

@@ -489,9 +489,8 @@ static irqreturn_t imgu_isr_threaded(int irq, void *imgu_ptr)
 			mutex_unlock(&imgu->lock);
 		} while (PTR_ERR(b) == -EAGAIN);
 
-		if (IS_ERR_OR_NULL(b)) {
-			if (!b || PTR_ERR(b) == -EBUSY)	/* All done */
-				break;
+		if (IS_ERR(b)) {
+			if (PTR_ERR(b) != -EBUSY)	/* All done */
 				dev_err(&imgu->pci_dev->dev,
 					"failed to dequeue buffers (%ld)\n",
 					PTR_ERR(b));