[media] cxusb: Use a dma capable buffer also for reading

Commit 17ce039b ("[media] cxusb: don't do DMA on stack")
added a kmalloc'ed bounce buffer for writes, but missed to do the same
for reads. As the read only happens after the write is finished, we can
reuse the same buffer.

As dvb_usb_generic_rw handles a read length of 0 by itself, avoid calling
it using the dvb_usb_generic_read wrapper function.
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/usb/dvb-usb/cxusb.c

@@ -59,23 +59,24 @@ static int cxusb_ctrl_msg(struct dvb_usb_device *d,
 			  u8 cmd, u8 *wbuf, int wlen, u8 *rbuf, int rlen)
 {
 	struct cxusb_state *st = d->priv;
-	int ret, wo;
+	int ret;
 
 	if (1 + wlen > MAX_XFER_SIZE) {
 		warn("i2c wr: len=%d is too big!\n", wlen);
 		return -EOPNOTSUPP;
 	}
 
-	wo = (rbuf == NULL || rlen == 0); /* write-only */
+	if (rlen > MAX_XFER_SIZE) {
+		warn("i2c rd: len=%d is too big!\n", rlen);
+		return -EOPNOTSUPP;
+	}
 
 	mutex_lock(&d->data_mutex);
 	st->data[0] = cmd;
 	memcpy(&st->data[1], wbuf, wlen);
-	if (wo)
-		ret = dvb_usb_generic_write(d, st->data, 1 + wlen);
-	else
-		ret = dvb_usb_generic_rw(d, st->data, 1 + wlen,
-					 rbuf, rlen, 0);
+	ret = dvb_usb_generic_rw(d, st->data, 1 + wlen, st->data, rlen, 0);
+	if (!ret && rbuf && rlen)
+		memcpy(rbuf, st->data, rlen);
 
 	mutex_unlock(&d->data_mutex);
 	return ret;