net: dsa: enable selftest support for all switches by default

Most of generic selftest should be able to work with probably all ethernet
controllers. The DSA switches are not exception, so enable it by default at
least for DSA.

This patch was tested with SJA1105 and AR9331.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>