sfc: Do not read STAT1.FAULT in efx_mdio_check_mmd()

This field does not exist in all MMDs we want to check, and all
callers allow it to be set (fault_fatal = 0).

Remove the loopback condition, as STAT2.DEVPRST should be valid
regardless of any fault.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>