usb: frmnet: Add support to use multiple transport functions

Current driver supports the single instance of control and data
transport. Add support to use multiple control and data transport
pairs per port (interface) basis. Control and data transport pair is
passed by user space from
/sys/class/android_usb/android0/f_rmnet/transports

Also moved and renamed common function str_to_xport and xport_to_str
to header file and updated respecitve callers of the function.

Change-Id: Ib0ceff85a1224336f7f4929235b6a40b8f13e849
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>