net: sfp: cope with SFPs that set both LOS normal and LOS inverted

net: sfp: cope with SFPs that set both LOS normal and LOS inverted

The SFP MSA defines two option bits in byte 65 to indicate how the
Rx_LOS signal on SFP pin 8 behaves:

bit 2 - Loss of Signal implemented, signal inverted from standard
        definition in SFP MSA (often called "Signal Detect").
bit 1 - Loss of Signal implemented, signal as defined in SFP MSA
        (often called "Rx_LOS").

Clearly, setting both bits results in a meaningless situation: it would
mean that LOS is implemented in both the normal sense (1 = signal loss)
and inverted sense (0 = signal loss).

Unfortunately, there are modules out there which set both bits, which
will be initially interpret as "inverted" sense, and then, if the LOS
signal changes state, we will toggle between LINK_UP and WAIT_LOS
states.

Change our LOS handling to give well defined behaviour: only interpret
these bits as meaningful if exactly one is set, otherwise treat it as
if LOS is not implemented.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1kyYQa-0004iR-CU@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>