ixgbe/base: add X550em link check

ixgbe/base: add X550em link check

This patch adds ixgbe_check_link_t_X550em for checking x550em
PHY link. We check that both the MAC and external x550em 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: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>