mailbox: msm_qmp: Assert on timeout

Dropped messages to AOP are generally fatal. The general rule of thumb
is to let clients decide whether the dropped message is fatal.

There have been some cases where there are messages sent to AOP as
part of the device shutdown, effectively overwriting the error state.

Add an assert to when QMP detects timeout so the error state on AOP
is preserved.

Change-Id: I5eaf42a76b1298246351ec9d66fe32e7308233bd
Signed-off-by: Jay Jayanna <jayanna@codeaurora.org>