net/mlx5e: Initialize CEE's getpermhwaddr address buffer to 0xff

Latest change in open-lldp code uses bytes 6-11 of perm_addr buffer
as the Ethernet source address for the host TLV packet.
Since our driver does not fill these bytes, they stay at zero and
the open-lldp code ends up sending the TLV packet with zero source
address and the switch drops this packet.

The fix is to initialize these bytes to 0xff. The open-lldp code
considers 0xff:ff:ff:ff:ff:ff as the invalid address and falls back to
use the host's mac address as the Ethernet source address.

Fixes: 3a6a931d ("net/mlx5e: Support DCBX CEE API")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>