libertas: implement suspend and resume core methods

We (ab)use priv->fw_ready to stop the worker thread from sending more
commands or data after the response to the HOST_SLEEP_ACTIVATE command
comes in. And we set it from the callback function _directly_ to ensure
that the worker thread sees it immediately; if we did it in
lbs_suspend() after waking up, that might be too late.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>