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>