staging: comedi: usbdux: remove impossible 'if (!devpriv)' check

The comedi core can only call this subdevice function is the private
data was successfully allocated during the (*auto_attach). Remove
the unnecessary check.

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