net: phy: sfp: Add labels to hwmon sensors

SFPs can report two different power values, the transmit power and the
receive power. Add labels to make it clear which is which. Also add
labels to the other sensors, VCC power supply, bias and module
temperature.

sensors(1) now shows:

sff2-isa-0000
Adapter: ISA adapter
VCC:          +3.23 V
temperature:  +33.4 C
TX_power:    276.00 uW
RX_power:     20.00 uW
bias:         +0.01 A

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>