machine/atsamd51: fix obvious bug in I2C code

I2C uses a hardcoded peripheral instead of referring to a specific
peripheral. In addition to that, it refers to the wrong SERCOM
(SERCOM3), which isn't used on any of the atsamd51 boards for I2C.

src/machine/machine_atsamd51.go

@@ -1008,7 +1008,7 @@ func (i2c I2C) WriteByte(data byte) error {
 	timeout := i2cTimeout
 	for !i2c.Bus.INTFLAG.HasBits(sam.SERCOM_I2CM_INTFLAG_MB) {
 		// check for bus error
-		if sam.SERCOM3_I2CM.STATUS.HasBits(sam.SERCOM_I2CM_STATUS_BUSERR) {
+		if i2c.Bus.STATUS.HasBits(sam.SERCOM_I2CM_STATUS_BUSERR) {
 			return errors.New("I2C bus error")
 		}
 		timeout--