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>