bnxt_en: Add async event handling for speed config changes.

On some dual port cards, link speeds on both ports have to be compatible.
Firmware will inform the driver when a certain speed is no longer
supported if the other port has linked up at a certain speed.  Add
logic to handle this event by logging a message and getting the
updated list of supported speeds.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>