cxgbe(4): Change the way t4_shutdown_adapter brings the link(s) down.

Modify the GPIO pins only on the Base-T cards and even there drive all
of them low instead of putting them in hi-z state. For the rest (this
is the common case), directly power off the PLLs of the high speed
serdes. This is the simplest method that does not involve or conflict
with the firmware but still works with all T4-T6 cards regardless of
what's plugged into the port.

This fixes a problem where the peer wouldn't always see a link down if
it is connected to the device using a -CR4 copper cable.

MFC after: 3 weeks
Sponsored by: Chelsio Communications