usb: gadget: fix a bug and a WARN_ON in dummy-hcd

This patch fixes a NULL pointer dereference and a WARN_ON in
dummy-hcd.  These things were the result of moving to the UDC core
framework, and possibly of changes to that framework.

Now unloading a gadget driver causes the UDC to be stopped after the
gadget driver is unbound, not before.  Therefore the "driver" argument
to dummy_udc_stop() can be NULL, so we must not try to print the
driver's name without checking first.

Also, the UDC framework automatically unregisters the gadget when the
UDC is deleted.  Therefore a sysfs attribute file attached to the
gadget must be removed before the UDC is deleted, not after.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>