sfc: Fix test for MDIO read failure

Commit 27dd2cac changed
mdio_clause45_check_mmds() to read both DEVS0 and DEVS1 registers and
to combine their values into an unsigned 32-bit mask.  This made the
following test for a negative (failure) value useless.  Fix it to
check whether either read failed.

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