USB: rework sysfs removal of interface files

Removing an interface's sysfs files before unregistering the interface
doesn't work properly, because usb_unbind_interface() will reinstall
altsetting 0 and thereby create new sysfs files.  This patch (as1074)
removes the files after the unregistration is finished.  It's not
quite as clean, but at least it works.

Also, there's no need to check if an interface has been registered
before removing its sysfs files.  If it hasn't been registered then
the files won't have been created, so usb_remove_sysfs_intf_files()
will simply do nothing.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>