msm: acpuclock: Fix acpuclk_data null pointer dereference

If acpuclk_data is null, dereferencing it in acpuclk_get_switch_time
causes a kernel panic.  Add pointer check to fix that.

Change-Id: I69b92671c8cf26cb45ec8f1193c147be2ebe4c16
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>