ixgbe: Fix KR to KX fail over for Mezzanine cards

This patch allows the recently added backplane device IDs that support KR
to fail over to KX during link setup.  This is accomplished by the new MAC
link setup function ixgbe_setup_mac_link_smartspeed().  Comments were also
updated to better document the reason for the delays chosen for KX, KX4, BX,
BX4 and KR connections.

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