diag: Create separate buffer for command responses

Diag driver was using an exisiting SMD Modem buffer for encoding
command responses. Make changes to add a new buffer to avoid
stomping on modem data.

Change-Id: I88ad0a2785dd8eef74f7a866bf9b3525d371e1f7
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>