net: phy: sfp: Handle unimplemented hwmon limits and alarms

Not all SFPs implement the registers containing sensor limits and
alarms. Luckily, there is a bit indicating if they are implemented or
not. Add checking for this bit, when deciding if the hwmon attributes
should be visible.

Fixes: 1323061a ("net: phy: sfp: Add HWMON support for module sensors")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>