comedi: vmk80xx: fix expression for tx buffer size

comedi: vmk80xx: fix expression for tx buffer size

commit ae2f65d3e8736b4d05b7e3eb4e5be7cae871a110 upstream.

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: f6d15c807df6 ("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.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>