sfc: Change falcon_probe_board() to fail for unsupported boards

The driver needs specific PHY and board support code for each SFC4000
board; there is no point trying to continue if it is missing.
Currently unsupported boards can trigger an 'oops'.

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