Stop using Red parameter for brightness value

This change corrects using Red parameter of RGBA for the Brightness value, to using the Alpha parameter of RGBA.

As the brightness max value for APA102 is `31` the `math.Min` stops the driver from using any value higher than 31.

drivers/spi/apa102.go

@@ -1,9 +1,9 @@
 package spi
 
 import (
-	"image/color"
-
 	"gobot.io/x/gobot"
+	"image/color"
+	"math"
 )
 
 // APA102Driver is a driver for the APA102 programmable RGB LEDs
@@ -93,7 +93,7 @@ func (d *APA102Driver) Draw() error {
 
 	for i, c := range d.vals {
 		j := (i + 1) * 4
-		tx[j] = 0xe0 + byte(c.R)
+		tx[j] = 0xe0 + byte(math.Min(c.A, 31))
 		tx[j+1] = byte(c.B)
 		tx[j+2] = byte(c.G)
 		tx[j+3] = byte(c.R)