sfc: Test for PHYXS faults whenever we cannot test link state bits

Depending on the loopback mode, there may be no pertinent link state
bits.  In this case we test the PHYXS RX fault bit instead.  Make
sure to do this in all cases where there are no link state bits.

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