staging: comedi: usbduxfast: remove impossible 'if (!devpriv)' checks

The private data 'devpriv' is allocated in the (*auto_attach) before
the subdevices are initialized and the (*auto_attach) has to complete
successfully before the core will call any of the subdevice operations.
Because of this the (!devpriv) checks in the subdevice code can never
fail. Remove them.

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>