usb: gadget: dummy-hcd: remember to update driver pointer

This patch (as1599) fixes dummy-hcd to make it update the appropriate
driver pointer when a new gadget driver is bound or unbound.  Without
this change, the gadget driver's name doesn't appear in dev_printk
output.

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