usb: gadget: Make USB_CONFIGFS_NCM buildable as a module

Change the Kconfig definition from bool to tristate to
allow building the NCM function as a loadable module.

Change-Id: Ia7637b7ae2fdd1809d3e3a6b1d4521ce6d136095
Signed-off-by: Jack Pham <jackp@codeaurora.org>