USB: at91_udc: Cleanup variables after failure in usb_gadget_register_driver()

This patch zeroes some variables when usb_gadget_register_driver()
fails. gadgetfs does a dummy registration to get the name of the USB
driver and then waits for user-land driver. If someone plugs the cable
in the meantime, bad things happen, because at91_udc has been left in
inconsistent state.

Signed-off-by: Wojtek Kaniewski <wojtekka@toxygen.net>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>