ixgbe: add link check for X550 copper

This patch adds ixgbe_check_link_t_X550em for checking copper PHY
link.  We check that both the MAC and external PHY have link. This
is to avoid a false link up between the internal and external PHY
when  the external PHY doesn't have link.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>