staging: comedi: fix checkpatch error

Fixed an error found by checkpatch.pl.
ERROR: space required after that ',' (ctx:VxV)
./drivers/ni_mio_common.c:3764

Signed-off-by: Geliang Tang <geliangtang@163.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>