travis: enable more of go vet shadow check

Now running:
  ! go vet -vettool=$(which shadow) ./... |& grep -v '"err"' | grep -e '^[^#]'

Which means that "err" aliasing is not checked, but others are. I may
enforce "err" aliasing later but it's more noisy so I'm not 100% sure
yet it's a good idea. Enforcing it for non-err variables looked like a
reasonable initial trade-off.

devices/bmxx80/bmx280smoketest/bmx280smoketest.go

@@ -55,7 +55,7 @@ func (s *SmokeTest) Run(f *flag.FlagSet, args []string) (err error) {
 		return err2
 	}
 	defer func() {
-		if err2 := i2cBus.Close(); err == nil {
+		if err2 = i2cBus.Close(); err == nil {
 			err = err2
 		}
 	}()
@@ -65,7 +65,7 @@ func (s *SmokeTest) Run(f *flag.FlagSet, args []string) (err error) {
 		return err2
 	}
 	defer func() {
-		if err2 := spiPort.Close(); err == nil {
+		if err2 = spiPort.Close(); err == nil {
 			err = err2
 		}
 	}()
@@ -139,7 +139,7 @@ func run(i2cBus i2c.Bus, i2cAddr uint16, spiPort spi.PortCloser) (err error) {
 		return err2
 	}
 	defer func() {
-		if err2 := i2cDev.Halt(); err == nil {
+		if err2 = i2cDev.Halt(); err == nil {
 			err = err2
 		}
 	}()
@@ -149,14 +149,14 @@ func run(i2cBus i2c.Bus, i2cAddr uint16, spiPort spi.PortCloser) (err error) {
 		return err2
 	}
 	defer func() {
-		if err2 := spiDev.Halt(); err == nil {
+		if err2 = spiDev.Halt(); err == nil {
 			err = err2
 		}
 	}()
 
 	i2cEnv := physic.Env{}
 	spiEnv := physic.Env{}
-	if err2 := i2cDev.Sense(&i2cEnv); err2 != nil {
+	if err2 = i2cDev.Sense(&i2cEnv); err2 != nil {
 		return err2
 	}
 	printEnv(i2cDev, &i2cEnv)

devices/ssd1306/ssd1306smoketest/ssd1306smoketest.go

@@ -75,7 +75,7 @@ func (s *SmokeTest) Run(f *flag.FlagSet, args []string) (err error) {
 		return err2
 	}
 	defer func() {
-		if err2 := i2cBus.Close(); err == nil {
+		if err2 = i2cBus.Close(); err == nil {
 			err = err2
 		}
 	}()

experimental/devices/mfrc522/commands/low_level.go

@@ -301,8 +301,8 @@ func (r *LowLevel) CardWrite(command byte, data []byte) ([]byte, int, error) {
 	n := byte(0)
 
 	for ; i > 0; i-- {
-		n, err := r.DevRead(CommIrqReg)
-		if err != nil {
+		var err error
+		if n, err = r.DevRead(CommIrqReg); err != nil {
 			return nil, -1, err
 		}
 		if n&(irqWait|1) != 0 {