sfc: Remove broken automatic fallback for invalid Falcon chip/board config

If the Falcon board config is invalid, we cannot proceed - we do not
have a valid board type to pass to falcon_probe_board(), and if we
kluge that to work with an unknown board then other initialisation
code will crash.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>