staging: comedi: ni_mio_common: Prefer 'unsigned int' to bare use of 'unsigned'

Fix the checkpatch.pl issues.

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>