diag: Clean diagfwd code

Remove diagfwd_bridge_init and _exit functions and call mhi_init
and _exit functions directly from mhi registration function
instead. Refactor other parts for more readability and readiness
of USB bridge implementation.

Change-Id: I8255d00da14137318b2135eb8e5f59ee49aa8c89
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>