usb: gadget: core: force synchronous registration

Registering a gadget driver is expected to complete synchronously and
immediately after calling driver_register() this function checks that
the driver has bound so as to return an error.

Set PROBE_FORCE_SYNCHRONOUS to ensure this is the case even when
asynchronous probing is set as the default.

Fixes: fc274c1e99731 ("USB: gadget: Add a new bus for gadgets")
Cc: [e-mail]
Signed-off-by: John Keeping <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/udc/core.c

···
 	driver->driver.bus = &gadget_bus_type;
 	driver->driver.owner = owner;
 	driver->driver.mod_name = mod_name;
+	driver->driver.probe_type = PROBE_FORCE_SYNCHRONOUS;
 	ret = driver_register(&driver->driver);
 	if (ret) {
 		pr_warn("%s: driver registration failed: %d\n",