bnxt_en: Fix ethtool autoneg logic.

1. Determine autoneg on|off setting from link_info->autoneg.  Using the
firmware returned setting can be misleading if autoneg is changed and
there hasn't been a phy update from the firmware.

2. If autoneg is disabled, link_info->autoneg should be set to 0 to
indicate both speed and flow control autoneg are disabled.

3. To enable autoneg flow control, speed autoneg must be enabled.

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