cxgbe(4): Block most access to the hardware as soon as the adapter stops.

Add a new hw_all_ok() routine and use it to avoid hardware access in the
public control interfaces (ifnet ioctls, ifmedia calls, etc.). Continue
to use hw_off_limits() in the private ioctls/sysctls and other debug
code. Retire adapter_stopped() as it's of no use by itself.

This fixes problems where ifnet slow-path operations would enter a
synch_op just before set_adapter_hwstatus(false) and touch the hardware
when it's not safe to do so.

MFC after: 1 week
Sponsored by: Chelsio Communications