diag: Create separate module for Diag-USB interaction

Diag USB structures are deeply coupled with diag drivers. Each
instance of USB channel is used explicitly. Make changes to
modularize USB channels designated for Diag.

Change-Id: Id408e227e7e92178afa847064afe18bb6c0dd019
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>