usb: gadget: f_ncm: add configfs support

Add configfs support to the NCM function driver so
that we can, eventually, get rid of kernel-based
gadget drivers.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>