s390/vtime: do not reset idle data on CPU hotplug

The sysfs attributes /sys/devices/system/cpu/cpu0/idle_count and
/sys/devices/system/cpu/cpu0/idle_time_us are reset to zero every
time a CPU is set online. The idle and iowait fields in /proc/stat
corresponding to idle_time_us are not reset. To make things
consistent do not reset the data for the sys attributes.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>