staging: comedi: plx9080.h: rename CamelCase enum value

Rename the enum value to fix the checkpatch.pl issue:
CHECK: Avoid CamelCase: <MARB_PCIv21_MODE>

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>