USB: cdc-acm: fix possible deadlock with multiple openers

The lock must be dropped before usb_autopm_interface_put() is called

Signed-off-by: Oliver Neukum <oliver@neukum.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

BUG=chrome-os-partner:3143
TEST=run power_Resume test in tight loops on Alex.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

Review URL: http://codereview.chromium.org/6820034

Change-Id: Ic0e9c134b803393c66dbec7d4a66e2b7af5ceae4