sfc: Wait for the link to stay up before running loopback selftest

It's been observed that some phys (such as the qt2025c) can
do down-up-down-up transitions, presumably as pcs block lock
settles down.

The loopback selftest will start sending data immediately
after the link comes up. Work around this by waiting for
the link state to stay up for two consecutive polls, rather
than one.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>