[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_gpio_tuner(struct dvb_usb_device *d,
		u8 addr, int onoff)
{
	u8  o[2] = {addr, onoff};
	u8  i;
	int rc;

	rc = cxusb_ctrl_msg(d, CMD_GPIO_WRITE, o, 2, &i, 1);

	if (rc < 0)
		return rc;
	if (i == 0x01)
		return 0;
	else {
		deb_info("gpio_write failed.\n");
		return -EIO;
	}
}

static int cxusb_d680_dmb_gpio_tuner(struct dvb_usb_device *d,
		u8 addr, int onoff)
{
	u8  o[2] = {addr, onoff};
	u8  i;
	int rc;

	rc = cxusb_ctrl_msg(d, CMD_GPIO_WRITE, o, 2, &i, 1);

	if (rc < 0)
		return rc;
	if (i == 0x01)
		return 0;
	else {
		deb_info("gpio_write failed.\n");
		return -EIO;
	}
}
