sfc: Fix check for failure of MC_CMD_FLUSH_RX_QUEUES

efx_mcdi_rpc_start() returns a negative value on error or zero on
success.  However one caller that can't properly handle failure then
does WARN_ON(rc > 0).  Change it to WARN_ON(rc < 0).

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