diag: Fix for sending responses in all modes

Diag was sending the responses only to USB with the change
9d92106b - 'diag: Create separate
buffer for command responses'. Make changes to send responses
to user space clients in non-USB modes and to USB in USB mode.

Change-Id: I6e36da23cb4ec99774fb5e4d72e4ab7cb1acb575
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>