mhi: core: Fix race condition on poll_inbound

Fix race condition in mhi_poll_inbound which could leave the
bounce buffer list and TRE list out of sync.

Change-Id: I76c7de75256b59f8a0dff2a85af733c441761385
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>
Signed-off-by: Tony Truong <truong@codeaurora.org>