diag: Re-organize HSIC/SMUX support

Until now, targets only had either SMUX or HSIC working. With
newer targets, one can have both interfaces working at same time.
This change re-organizes diag code, so that multiple interfaces
can work at the same time.

Change-Id: If201d268ab1ea66c4d420ea6de460e87f1fa81e9
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>