staging: comedi: usbduxsigma: tidy up chanToInterval()

Rename the function to have namespace associated with the driver.
Rename the CamelCase parameter 'nChannels' to avoid the checkpatch.pl
warning.

Tidy up the function a bit.

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>