usb: diag_bridge: Rename driver and functions for clarity

This will hopefully disambiguate naming clashes between this bridge
driver and existing DIAG drivers.

* Rename diag_usb.c to diag_bridge.c
* Rename diag_{open,close,read,write} to diag_bridge_*
* Rename Kconfig symbol
* Fix module build

Change-Id: Ia5b4884a7d85e2fa8f8c3865044a074a51e84adc
Signed-off-by: Jack Pham <jackp@codeaurora.org>