USB: cdc-acm: fix deadlock in acm_tty_open

The mutex must be dropped before calling usb_autopm_interface_put,
else a concurrent call to acm_suspend will trigger a deadlock.

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/6824041

Change-Id: I35633cfd0aad8f9b3d72c83aefb25e2ed46e1a08