bnxt_en: Add async. event logic for PHY configuration changes.

If the link settings have been changed by another function sharing the
port, firmware will send us an async. message.  In response, we will
call the new bnxt_init_ethtool_link_settings() function to update
the current settings.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>