mhi: uci: Fix race conditon in mhi_poll_inbound

A race condition exists in MHI_UCI whereby a client could receive
a buffer, inconsistent with the return code from MHI.

Change-Id: I4a932ea608ce967c1588f092c60d03747bad2064
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>