staging: comedi: ni_mio_common: fix build warning

The kbuild test robot detected a build warning causes by commit f878071a.

>> drivers/staging/comedi/drivers/ni_mio_common.c:2274:34:
   warning: right-hand operand of comma expression has no effect [-Wunused-value]

That line should have been terminated by ';' and the following line removed.
Not sure why it even builds on my test system...

Reported-by: kbuild test robot
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>