staging: comedi: usbduxsigma: tidy up usbdux_ai_inttrig()

Rename the function so it has namespace associated with the driver.
Rename the local variable used for the private data pointer.

Remove the dev_err() messages, they are just added noise.

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>