dm: net: macb: Update macb_linkspd_cb() signature

This updates DM version macb_linkspd_cb() signature for future
expansion, eg: adding an implementation for link speed changes.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>