ixgbe: fix ethtool -A|a behavior

We were basicly ignoring ethtool users request for FC autoneg
and replying to queries with a "best guess".  This patch
enables the driver to store if we want to enable/disable
autoneg FC and do the correct behavior.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>