diag: Make fixes to diag-MHI interaction

Diag driver was handling the close of MHI channels asynchronously.
Also, buffers that were mapped to DMA were not unmapped at right
instances. This patch fixes those issues.

Change-Id: I285fc901c730ef9e3a89cc7211289f362f7eb2e4
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>