diag: Disable the code for diag_smux

Disable the diag_smux code to fix the build with
diagfwd USB enabled since the dependencies for
diag_smux are not available it is deprecated in
this kernel version.
Remove the diagfwd_smux driver as it is no longer in use.
Clean the code wherever possible.

Change-Id: I6a8bc3a34f67ed5b1ca175cad0b3067e2c2df199
Signed-off-by: Gustavo Solaira <gustavos@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>