ixgbe/base: fix init handling of X550em link down

ixgbe/base: fix init handling of X550em link down

This patch changes the return value for ixgbe_setup_internal_phy_t_x550em
when link is down to IXGBE_SUCCESS.
The driver will call ixgbe_setup_internal_phy_t_x550em when a link status
change is reported. The links status change can occur on link up or link
down, and if the link status change is for link down then there is no iXFI
setup necessary and no error condition needs to be returned.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>