net: phy: sfp: hwmon: Fix scaling of RX power

The RX power read from the SFP uses units of 0.1uW. This must be
scaled to units of uW for HWMON. This requires a divide by 10, not the
current 100.

With this change in place, sensors(1) and ethtool -m agree:

sff2-isa-0000
Adapter: ISA adapter
in0:          +3.23 V
temp1:        +33.1 C
power1:      270.00 uW
power2:      200.00 uW
curr1:        +0.01 A

        Laser output power                        : 0.2743 mW / -5.62 dBm
        Receiver signal average optical power     : 0.2014 mW / -6.96 dBm

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