ixgbe: Remove MAC X550EM_X 1Gbase-t led_[on|off] support

Since FW configures the PHY and MAC X550EM_X has no
PHY access, led_[on|off] is not supported with the 1Gbase-t design.

Removed MAC X550EM_X 1Gbase-t led_[on|off] support by setting
function pointers to NULL and added NULL pointer checks. Also set
init_led_link_act to NULL and added NULL pointer check.

Signed-off-by: Paul Greenwalt <paul.greenwalt@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>