clk: qcom: clk-aop-qmp: pass mbox pointer by reference in qmp_update_client

Pass the mbox pointer to qmp_update_client() by reference so that
it is possible to update its value inside of the function.

Change-Id: Iefa4587f6e77b6e7029ed07236fa3a51ca09c129
Signed-off-by: David Collins <collinsd@codeaurora.org>