tcm_usb_gadget: Fix nexus leak

This patch adds the missing tcm_usbg_drop_nexus() to properly
release tcm_usbg_nexus memory during typical ->fabric_drop_tpg()
callback shutdown.

Reported-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
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>