diag: Modularize different logging modes

Diag driver has an ability to log diag data to USB or to the user
space clients. Make changes to modularize different modes of
logging schemes.

Change-Id: I91c95ce4ec8117f6926d8e6dfa164fd6cb07dc4b
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>