usb: f_rmnet: Add support for BAM DMUX data path

Add support for RMNET and DPL functions to work in SW mode using
the BAM DMUX transport driver.

Also clean up the BAM2BAM support from u_bam_dmux.c along with
fixing compilation issues. Also edit the Makefile to compile
BAM DMUX driver.

Change-Id: I5887f4fd599fcd82c3567be3972ad4248c7b7a23
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>