clk: qcom: clk-aop-qmp: correct mailbox channel request logic

Ensure that the clk-aop-qmp probe function correctly returns
-EPROBE_DEFER if mbox_request_channel() returns -EPROBE_DEFER.
This avoids invalid memory dereferencing during QDSS clock
calls in the case that the AOP mailbox is not yet available.

Change-Id: I4ef07ceaace05a1911f51992a181903b4365afe2
Signed-off-by: David Collins <collinsd@codeaurora.org>