soc: qcom: ipc_router: Update transport name from HSIC to USB

Currently the ipc_router_hsic_xprt transport driver which
interfaces with available USB endpoints for IPC message exchange,
uses "hsic" in the code and comments.
Update this string to "usb" and make similar change to filename,
Makefile and Documentation.

Change-Id: I37f8f8dafd85bfde300804b36f1b6f20ed565352
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>