diag: Add support for mutiple HSIC instances

On a future target, applications processor will talk to
two MDMs simultaneously. This change will enable multiple
HSIC connections towards this configuration.

Change-Id: I2138b1d458936eddb8276a12e92916a4893d922b
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>