USB: gadget: Rename usb_gadget_probe_driver()

In preparation for adding a "gadget" bus, this patch renames
usb_gadget_probe_driver() to usb_gadget_register_driver().  The new
name will be more accurate, since gadget drivers will be registered on
the gadget bus and the probing will be done by the driver core, not
the UDC core.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/YmSc29YZvxgT5fEJ@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>