ixgbe: improve log about autoneg being disabled

On ESXi OS, when user disables auto negotiation, the following log
appears: "(unsupported) Flow control autoneg is disabled".
It is true that auto negotiation is disabled but it is
not necessarily true that it is not supported.

Signed-off-by: Jakub Chylkowski <jakubx.chylkowski@intel.com>
Signed-off-by: Guinan Sun <guinanx.sun@intel.com>
Reviewed-by: Wei Zhao <wei.zhao1@intel.com>

Approved by: imp
Obtained from: DPDK (ab6ac48d483ef7f906b90f45182f2ddf3254d876)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31621