ina219: Interpret signed output register values (#54)

The current and shunt voltage register values are signed, twos
complement format. In my tests interpreting them directly in the
way implemented here gave output consistent with other ina219
libraries.

Co-authored-by: Eldon Stegall <periphgithub@eldondev.com>

ina219/ina219.go

@@ -110,7 +110,7 @@ func (d *Dev) Sense() (PowerMonitor, error) {
 		return PowerMonitor{}, errReadShunt
 	}
 	// Least significant bit is 10µV.
-	pm.Shunt = physic.ElectricPotential(shunt) * 10 * physic.MicroVolt
+	pm.Shunt = physic.ElectricPotential(int16(shunt)) * 10 * physic.MicroVolt
 
 	bus, err := d.m.ReadUint16(busVoltageRegister)
 	if err != nil {
@@ -128,7 +128,7 @@ func (d *Dev) Sense() (PowerMonitor, error) {
 	if err != nil {
 		return PowerMonitor{}, errReadCurrent
 	}
-	pm.Current = physic.ElectricCurrent(current) * d.currentLSB
+	pm.Current = physic.ElectricCurrent(int16(current)) * d.currentLSB
 
 	power, err := d.m.ReadUint16(powerRegister)
 	if err != nil {