usb/gadget: Constify usb_gadget_get_string "table" argument

The table is never modified by the function. This allows us
to use it on a statically defined table that is marked const.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>