usb: gadget: f_tcm: add configfs support

Allow using the tcm function as a component of a gadget composed with
ConfigFS.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>