USB: auerswald: fix file release handler

If this down_interruptible() does fail due to signal_pending() then the state
of the driver will get trashed in interesting ways, because userspace cannot
and will not retry the close().

Cc: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>