USB: g_printer: fix bug in unregistration

This patch (as1441) fixes a bug in g_printer.  The gadget driver, char
device number, and class device should be unregistered in reverse
order of registration.  As it is now, when the module is unloaded the
class device gets unregistered first, causing a crash when the unbind
method tries to access it.

This fixes Bugzilla #25882.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Roland Kletzing <devzero@web.de>
CC: Craig W. Nadler <craig@nadler.us>
CC: <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>