mailbox: msm_qmp: Fix error handling for mailbox framework

If AOP fails to read a packet in a timely manner, reset the size
to 0 so messages can be sent again. Fix the error code to ETIME
which is handled by the mailbox framework.

Change-Id: Iebe8e9370ef3c6dd63e6a06309c975ddfe459861
Signed-off-by: Chris Lew <clew@codeaurora.org>