staging: comedi: ni_660x: checkpatch.pl cleanup (trailing whitespace)

Fix the checkpatch.pl error:

ERROR: trailing whitespace
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>

drivers/staging/comedi/drivers/ni_660x.c

@@ -28,7 +28,7 @@
  *
  * Encoders work.  PulseGeneration (both single pulse and pulse train)
  * works.  Buffered commands work for input but not output.
- * 
+ *
  * References:
  * DAQ 660x Register-Level Programmer Manual  (NI 370505A-01)
  * DAQ 6601/6602 User Manual (NI 322137B-01)