staging: comedi: usbduxsigma: return void from usbduxsigma_pwm_pattern()

This function never fails and currently returns '1' so that the
usbduxsigma_pwm_write() function returns to proper value (insn->n).

It's also called by usbduxsigma_pwm_config() which also returns the
'1' value. The proper return code for this function should be '0' to
indicate success.

Change the return type of the function to void and just have the
callers return the proper values.

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>