ixgbe: Disable EEE for backplane X550EM_X

From Zach:
Intel documentation indicates that backplane X550EM_X KR devices do not
support Energy Efficient Ethernet. Prior to this patch, X552 devices
(device ID 0x15AB) will crash the system when transitioning EEE state
via sysctl.

Signed-off-by: Zach Vargas <zvargas@xes-inc.com>

PR: 240320
Submitted by: Zach Vargas <zvargas@xes-inc.com>
Reviewed by: erj@
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D21673