net: rmnet_usb: Remove mux support from data path

Higher layer driver takes care of muxing and dmuxing
of IP packet on data path. Hence embedded rmnet does
not require to support muxing and dmuxing for data
path. This requires restructuring in rmnet control
driver as it is depending upon rmnet data driver to
support muxing on control path.

Change-Id: Ie354f5e63e7c00e06bd017395daf4d46f9667aff
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>