USB: diag: Add stubs for USB-DIAG API

Diag char driver can support SD card, UART and socket transports
apart from USB.  Add stubs for USB-DIAG API to make diag char
independent of USB. Remove unused diag_read_from_cb().

CRs-Fixed: 423153
Change-Id: Id1562f9738b6f0cf5434e4dd54ce943373d1322f
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>