usb: gadget: f_ncm: use usb_gstrings_attach

Trivial patch making use of the new usb_gstrings_attach
interface.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>