gpio: fix a bug which prevented digital pins reads from working correctly

when reading from the value file, it is important to seek back to the front
so that we get the fresh values from the kernel driver for every new read

digitalpin.go

@@ -117,6 +117,7 @@ func (p *digitalPin) Read() (int, error) {
 	if _, err := p.val.Read(buf); err != nil {
 		return 0, err
 	}
+	p.val.Seek(0, 0)
 	var val int
 	if buf[0] == '1' {
 		val = 1