diag: Add snapshot of Diag driver

This is a snapshot of the diag driver taken from msm-4.9 commit
<34c89b3303f> (Merge "icnss: Do not assert if reboot requested during
PDR").

Fix indentation and style warnings. Simplify HDLC decoding loop.

Change-Id: I42f402aa1f9b340c7ef8dccc30614f01effcb50c
Signed-off-by: Chris Lew <clew@codeaurora.org>