ixgbe: fix setting and reporting of advertised speeds

Add the ability to set 100/F on x540.
Fix reporting of advertised modes by adding check for phy.autoneg_advertised

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>