USB: testusb: testusb compatibility with FunctionFS gadget

The FunctionFS gadget may provide the source/sink interface
not as the first interface (with id == 0) but some different
interface hence a code to find the interface number is
required.

(Note that you will still configure the gadget to report
idProduct == 0xa4a4 (an "echo 0xa4a4
>/sys/module/g_ffs/parameters/usb_product" should suffice) or
configure host to handle 0x0525:0xa4ac devices using the
usbtest driver.)

Signed-off-by: Michal Nazarewicz <[email protected]>
Cc: Kyungmin Park <[email protected]>
Cc: Marek Szyprowski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>