Fix dcPin method call in tx function

Author: ysoldak

ssd1306/ssd1306_spi.go

@@ -62,7 +62,7 @@ func (b *SPIBus) flush() error {
 // tx sends data to the display
 func (b *SPIBus) tx(data []byte, isCommand bool) error {
 	b.csPin.High()
-	b.dcPin.Set(!isCommand)
+	b.dcPin(!isCommand)
 	b.csPin.Low()
 	err := b.wire.Tx(data, nil)
 	b.csPin.High()