staging: comedi: usbduxfast: tidy up usbduxfast_ai_stop()

Remove the devpriv sanity check, all the callers have previously
verified that its valid.

Absorb usbduxfast_unlink_urbs() into this function since it's the
only caller and the only thing it does is call usb_kill_urb().

This function will always succeed so just return 0 and remove the
local variable 'ret'.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>