cxgb3 - Fix potential MAC hang

Under rare conditions, the MAC might hang while generating a pause frame.
This patch fine tunes the MAC settings to avoid the issue, allows for
periodic MAC state check, and triggers a recovery if hung.

Also fix one MAC statistics counter for the rev board T3B2.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>