bnxt_en: Don't allow autoneg on cards that don't support it.

Some cards do not support autoneg.  The current code does not prevent the
user from enabling autoneg with ethtool on such cards, causing confusion.
Firmware provides the autoneg capability information and we just need to
store it in the support_auto_speeds field in bnxt_link_info struct.
The ethtool set_settings() call will check this field before proceeding
with autoneg.

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