s390/cio: export CMG value as decimal

Change format of the "cmg" sysfs attribute from hex to decimal to
make it easier to consume. Note that this should not break any existing
users since only values 2 and 3 are currently exported. Also the main
user already assumes decimal notation [1].

[1] https://sourceforge.net/p/sblim/gather/ci/master/tree/plugin/metriczCH.c



Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>