drivers: qcom: rpm_stats: Fix memory leak

The @privdata memory allocated is not freed at the end of the function.
There is no need for this memory to be in heap. Allocate the variable in
stack.

Change-Id: Id18917194b657bf43e35181106f914d4b2c3055c
Signed-off-by: Lina Iyer <ilina@codeaurora.org>