[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_mt352_frontend_attach(struct dvb_usb_adapter *adap)
{
	/* used in both lgz201 and th7579 */
	if (usb_set_interface(adap->dev->udev, 0, 0) < 0)
		err("set interface failed");

	cxusb_ctrl_msg(adap->dev, CMD_DIGITAL, NULL, 0, NULL, 0);

	adap->fe_adap[0].fe = dvb_attach(mt352_attach, &cxusb_mt352_config,
					 &adap->dev->i2c_adap);
	if ((adap->fe_adap[0].fe) != NULL)
		return 0;

	return -EIO;
}

static int cxusb_mt352_frontend_attach(struct dvb_usb_adapter *adap)
{
	/* used in both lgz201 and th7579 */
	if (usb_set_interface(adap->dev->udev, 0, 0) < 0)
		err("set interface failed");

	cxusb_ctrl_msg(adap->dev, CMD_DIGITAL, NULL, 0, NULL, 0);

	adap->fe_adap[0].fe = dvb_attach(mt352_attach, &cxusb_mt352_config,
					 &adap->dev->i2c_adap);
	if ((adap->fe_adap[0].fe) != NULL)
		return 0;

	return -EIO;
}
