USB: sierra: fix AA deadlock in open error path

commit 353fe198602e8b4d1c7bdcceb8e60955087201b1 upstream.

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")

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>