tcm_usb_gadget: Don't strip off nexus WWPN prefix

Avoid stripping off the 'naa.' I_T nexus prefix from configfs
attribute store input, so that user-space will get back what
it originaly wrote into ../usb_gadget/$WWPN/$TPGT/nexus.

Note the SCSI initiator WWPN is purely symbolic for UAS + BOT,
so it will not effect host side code.
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>

drivers/usb/gadget/legacy/tcm_usb_gadget.c

@@ -1657,7 +1657,7 @@ static ssize_t tcm_usbg_tpg_nexus_store(struct config_item *item,
 	if (i_port[strlen(i_port) - 1] == '\n')
 		i_port[strlen(i_port) - 1] = '\0';
 
-	ret = tcm_usbg_make_nexus(tpg, &i_port[4]);
+	ret = tcm_usbg_make_nexus(tpg, &i_port[0]);
 	if (ret < 0)
 		return ret;
 	return count;