net: sfp: report error on failure to read sfp soft status

Report a rate-limited error if we fail to read the SFP soft status,
and preserve the current status in that case. This avoids I2C bus
errors from triggering a link flap.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>