sfc: SFT9001: Add cable diagnostics

The SFT9001 firmware implements cable diagnostics; run those and
include their results in a self-test.  In case of a cable fault, do
not fail the self-test as a whole; only faults in the NIC should cause
that.

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