USB: fix spinlock recursion in cdc-acm.c

this fixes the spinlock recursion issue. The older fix was incomplete.


Signed-off-by: Oliver Neukum <oneukum@suse.de>
Acked-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>