cpufreq: Add if cpu is online check in show

Make sure CPU is online before proceeding with any "show"
ops. Without this check, the show can race with hotplug
and try to show the details of a stale or non-existent
policy.

CRs-Fixed: 689522
Change-Id: Ie791c73cb281bcfc4d722f7c8c10eee07cb11f2e
Signed-off-by: Maria Yu <aiquny@codeaurora.org>