comedi: vmk80xx: fix expression for tx buffer size

The expression for setting the size of the allocated bulk TX buffer
(`devpriv->usb_tx_buf`) is calling `usb_endpoint_maxp(devpriv->ep_rx)`,
which is using the wrong endpoint (should be `devpriv->ep_tx`).  Fix it.

Fixes: a23461c4 ("comedi: vmk80xx: fix transfer-buffer overflow")
Cc: Johan Hovold <johan@kernel.org>
Cc: stable@vger.kernel.org # 4.9+
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20220607171819.4121-1-abbotti@mev.co.ukSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/comedi/drivers/vmk80xx.c

@@ -684,7 +684,7 @@ static int vmk80xx_alloc_usb_buffers(struct comedi_device *dev)
 	if (!devpriv->usb_rx_buf)
 		return -ENOMEM;
 
-	size = max(usb_endpoint_maxp(devpriv->ep_rx), MIN_BUF_SIZE);
+	size = max(usb_endpoint_maxp(devpriv->ep_tx), MIN_BUF_SIZE);
 	devpriv->usb_tx_buf = kzalloc(size, GFP_KERNEL);
 	if (!devpriv->usb_tx_buf)
 		return -ENOMEM;