sfc: work around TRIGGER_INTERRUPT command not working on SFC9140

MC_CMD_TRIGGER_INTERRUPT does not work on the SFC9140, as used in the
sfn7x42q and sfn7x24f.
Check for this using the MCDI workaround mechanism.
The command is only used during self test.  If it's not supported, skip
the interrupt test.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>