drivers: qcom: rpmh_master_stat: Remove unused size for SMEM item

Size is not used in stats driver after reading item from qcom_smem_get().
It can be passed as NULL if the caller of this API don't intend to use
it anywhere. Update driver to remove unused variable.

Change-Id: I60fe4e193fff4366eda0f29c684806421e9e6b86
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>