eni: Fix data race when updating limits
eni: Fix data race when updating limits

Fixes:
```
WARNING: DATA RACE
Write at 0x00c0005b0750 by goroutine 308:
  runtime.mapassign_faststr()
      /usr/local/go/src/runtime/map_faststr.go:202 +0x0
  github.com/cilium/cilium/pkg/aws/eni.UpdateLimitsFromUserDefinedMappings()
      /home/vagrant/go/src/github.com/cilium/cilium/pkg/aws/eni/limits.go:269 +0xdf
  github.com/cilium/cilium/pkg/aws/eni.(*ENISuite).TestUpdateLimitsFromUserDefinedMappings()
      /home/vagrant/go/src/github.com/cilium/cilium/pkg/aws/eni/limits_test.go:47 +0x11d
  runtime.call32()
      /usr/local/go/src/runtime/asm_amd64.s:539 +0x3a
  reflect.Value.Call()
      /usr/local/go/src/reflect/value.go:321 +0xd3
  gopkg.in/check%2ev1.(*suiteRunner).forkTest.func1()
      /home/vagrant/go/src/github.com/cilium/cilium/vendor/gopkg.in/check.v1/check.go:781 +0xa0a
  gopkg.in/check%2ev1.(*suiteRunner).forkCall.func1()
      /home/vagrant/go/src/github.com/cilium/cilium/vendor/gopkg.in/check.v1/check.go:675 +0xd9
```

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

cilium-team/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium-team/cilium