usb: gadget: f_mdm_data: Add function driver support

Add configfs APIs to f_mdm_data.c driver to enable it to be used
as a function driver. Keep generic support for both IN and OUT
EPs for extensibility.
While at this, also cleanup appendix of ZLP after each request.
Refactor/Cleanup code wherever applicable.

Change-Id: I284a0bdf69ae41b3144a9ce1df494bad4d0f76a6
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>