sfc_ef100: check firmware version at start-of-day

Early in EF100 development there was a different format of event
 descriptor; if the NIC is somehow running the very old firmware
 which will use that format, fail the probe.

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