sfc: Add MC BISTs to ethtool offline self test on EF10

To run BISTs the MC goes down in to a special mode where it will only
respond to MCDI from the testing PF, and TX, RX and event queues are
torn down. Other PFs get a message as it goes down to tell them it's
going down.

When the other PFs get this message, they check the soft status
register to tell when the MC has rebooted after BIST mode and they can
start recovery.

[bwh: Convert the test result to 1 or -1 as for earlier NICs]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>