usb: gadget: hid: fix error code in do_config()

Return an error code if usb_get_function() fails.  Don't return success.

Fixes: 4bc8a33f ("usb: gadget: hid: convert to new interface of f_hid")
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211011123739.GC15188@kili


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>