[media] tm6000: remove needless check

Smatch reports a warning:
	drivers/media/usb/tm6000/tm6000-video.c:646 tm6000_prepare_isoc() error: we previously assumed 'dev->urb_buffer' could be null (see line 624)

This is not really a problem, but it actually shows that the check
if urb_buffer is NULL is being done twice: at the if and at
tm6000_alloc_urb_buffers().

We don't need to do it twice. So, remove the extra check. The code
become cleaner, and, as a collateral effect, smatch becomes happy.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/usb/tm6000/tm6000-video.c

@@ -621,7 +621,7 @@ static int tm6000_prepare_isoc(struct tm6000_core *dev)
 		    dev->isoc_in.maxsize, size);
 
 
-	if (!dev->urb_buffer && tm6000_alloc_urb_buffers(dev) < 0) {
+	if (tm6000_alloc_urb_buffers(dev) < 0) {
 		tm6000_err("cannot allocate memory for urb buffers\n");
 
 		/* call free, as some buffers might have been allocated */