usb: gadget: rename gadget transport type

Modify the transport_type enum and rename file to reflect
all supported gadget transport types not just serial. Remove
reference to the old file name from some of board files and
device files, as they are no longer dependent on it.

Change-Id: Ic1a07382fff71e5aa2f9473a95eab8da1330b82f
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>