ixgbe: support link speed auto-negotiation on X550em_x

ixgbe: support link speed auto-negotiation on X550em_x

Normally the auto-negotiation is supported by FW. SW need not care about
that. But on x550em_x, FW doesn't support auto-neg. As the x550em_x ports
are 10G, if we connect the port will a peer which is 1G, the link will
always be down.
We need support auto-neg by SW to avoid this link down issue. As we already
have the code to handle the link speed setting, what we need is a trigger.
When the advertised link speed changes, a PHY interruption will be
triggered. So, we should handle this interrupt and call ixgbe_handle_lasi
to set the link speed correctly.

Please be aware it's working when auto-neg is on. If the auto-neg of the
peer port is turned off and its speed is indicated manually, we should also
set the speed of our own port manually.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Shaopeng He <shaopeng.he@intel.com>