sfp: improve RX_LOS handling

There are two bits in the option word for the RX_LOS signal.  One
reports that the RX_LOS signal is active high, the other reports that
it is active low.  When both or neither are set, the result is not
well defined in the specification.

Rather than assuming that neither set means normal RX_LOS, take this
as meaning no RX_LOS signal available, thereby ignoring the signal.

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