USB: dummy_hcd: don't register drivers on the platform bus

This patch (as1017) makes dummy_hcd behave more like the other USB
peripheral controller drivers by no longer registering its
gadget driver on the platform bus.  Doing that has always been a
mistake, since a usb_gadget_driver isn't a platform_driver.  Instead
the gadget driver is left unregistered in sysfs.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>