diag: Handle error case and fix compiler warnings

Currently, if write to PCIE fails then diag is not
freeing the buffers due to which not able to queue
next read from peripherals.
Also fix warnings that are related to diag over pcie
communication path.

Change-Id: Id6cf903aff7510a9256eac8f3b6da036cbde0587
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>