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: Ie31bd7d57aea0ca05ba7f8ad1bf529f8836a7104
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
test ath6kl

Change-Id: Id39a4bd5e9d78b4902bec080f7ec70c244b167e8