net: selftest: fix build issue if INET is disabled

In case ethernet driver is enabled and INET is disabled, selftest will
fail to build.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: 3e1e58d64c3d ("net: add generic selftest support")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210428130947.29649-1-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net: add generic selftest support

Port some parts of the stmmac selftest and reuse it as basic generic selftest
library. This patch was tested with following combinations:
- iMX6DL FEC -> AT8035
- iMX6DL FEC -> SJA1105Q switch -> KSZ8081
- iMX6DL FEC -> SJA1105Q switch -> KSZ9031
- AR9331 ag71xx -> AR9331 PHY
- AR9331 ag71xx -> AR9331 switch -> AR9331 PHY

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