staging: comedi: usbdux: absorb usbdux_attach_common into caller

This function is only called by usbdux_auto_attach(), absorb it.

Also, there is no reason to down/up the semaphore during the attach.
None of the subdevices are functioning yet so there are no commands
being sent to the usb device.

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>