USB: sierra: fix AA deadlock in open error path

Fix AA deadlock in open error path that would call close() and try to
grab the already held disc_mutex.

Fixes: b9a44bc1 ("sierra: driver urb handling improvements")

Cc: <stable@vger.kernel.org>	# v2.6.31
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>