USB: UDC: net2280: Fix memory leaks

As Anton and Evgeny have noted, the net2280 UDC driver has a problem
with leaking memory along some of its failure pathways.  It also has
another problem, not previously noted, in that some of the failure
pathways will call usb_del_gadget_udc() without first calling
usb_add_gadget_udc_release().  And it leaks memory by calling kfree()
when it should call put_device().

Previous attempts to fix the problems have failed because of lack of
support in the UDC core for separately initializing and adding
gadgets, or for separately deleting and freeing gadgets.  The previous
patch in this series adds the necessary support, making it possible to
fix the outstanding problems properly.

This patch adds an "added" flag to the net2280 structure to indicate
whether or not the gadget has been registered (and thus whether or not
to call usb_del_gadget()), and it fixes the deallocation issues by
calling usb_put_gadget() at the appropriate point.

A similar memory leak issue, apparently never before recognized, stems
from the fact that the driver never initializes the drvdata field in
the gadget's embedded struct device!  Evidently this wasn't noticed
because the pointer is only ever used as an argument to kfree(), which
doesn't mind getting called with a NULL pointer. In fact, the drvdata
for gadget device will be written by usb_composite_dev structure if
any gadget class is loaded, so it needs to use usb_gadget structure
to get net2280 private data.

CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported-by: Anton Vasilyev <vasilyev@ispras.ru>
Reported-by: Evgeny Novikov <novikov@ispras.ru>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>