net: dsa: pass extack to dsa_switch_ops :: port_mirror_add()

Drivers might have error messages to propagate to user space, most
common being that they support a single mirror port.

Propagate the netlink extack so that they can inform user space in a
verbal way of their limitations.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>