staging: comedi: vmk80xx: zero buffers on allocation

Zero out `devpriv->usb_tx_buf` and `devpriv->usb_rx_buf` on allocation.
When sending data to the USB device, this ensures any unused part of the
buffer will not contain random crap.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>