usb: gadget: f_tcm: use usb_gstrings_attach

Do not directly use file static strings definitions in instances of f_tcm.
Instead use usb_gstrings_attach.

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>