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

Commit 17ce039b4e54 ("[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>
static int cxusb_d680_dmb_streaming_ctrl(
		struct dvb_usb_adapter *adap, int onoff)
{
	if (onoff) {
		u8 buf[2] = { 0x03, 0x00 };
		cxusb_d680_dmb_drain_video(adap->dev);
		return cxusb_ctrl_msg(adap->dev, CMD_STREAMING_ON,
			buf, sizeof(buf), NULL, 0);
	} else {
		int ret = cxusb_ctrl_msg(adap->dev,
			CMD_STREAMING_OFF, NULL, 0, NULL, 0);
		return ret;
	}
}

static int cxusb_d680_dmb_streaming_ctrl(
		struct dvb_usb_adapter *adap, int onoff)
{
	if (onoff) {
		u8 buf[2] = { 0x03, 0x00 };
		cxusb_d680_dmb_drain_video(adap->dev);
		return cxusb_ctrl_msg(adap->dev, CMD_STREAMING_ON,
			buf, sizeof(buf), NULL, 0);
	} else {
		int ret = cxusb_ctrl_msg(adap->dev,
			CMD_STREAMING_OFF, NULL, 0, NULL, 0);
		return ret;
	}
}
