diag: usb: Add support for partial writes over USB

Diag driver needs to support writing partial packets over USB to
remove any dependency on the controller properties. Make change
to write large buffers in chunks. Also add the support for the
maximum packet size usb supports.

Change-Id: I9f70396adbe3dfb988ad65319442ede88fbc6896
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>