msm: acpuclock: Add a null check for acpuclk_data

If acpuclock fails to probe before a client calls acpuclk_get_rate,
a
null dereference will occur. Fix this by adding the
appropriate
checks.

Change-Id:
If873a64711e9cc807b1734996f7c9358bb08161a
Signed-off-by: Vikram
Mulukutla <markivx@codeaurora.org>

idea is taken here -
https://github.com/jrior001/android_kernel_samsung_d2/commit/a24b75296b5e735966b1d95b51882c65d1037080