usb: gadget: let f_* use usb_string_ids_tab() where it makes sense

Instead of calling usb_string_id() multiple times I replace it with one
usb_string_ids_tab(). The NULL pointer in struct usb_string with "" and
are not overwritten in fail or unbind case.

The conditional assignment remains because some gadgets recycle the string
ID because the same descriptor (and string ID) is used if we have more
than one config descriptor.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>