sfc: Make handling of MC reboot more reliable

When the MC reboots, either as part of a firmware upgrade or due to a
bug, it attempts to complete (with an error) any requests that were
outstanding before the reboot.  Since there is an inherent race
condition in checking this, it will also write to a status word in
shared memory.

If we look at each of these separately, we may detect each reboot
twice, resulting in a spurious command failure after a firmware
upgrade or frustrating recovery from a firmware bug.  Instead, if a
request completion indicates a reboot, we must poll and clear the
status word.

This bug was previously masked by use of an incorrect address for the
status word.  Fix that, using the definition now included in
mcdi_pcol.h.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>