continues executing after operateOnPort in iec eip

[ShiChangkuo]

fixes #857

the testing result as follows:

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccIecEIPResource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccIecEIPResource_basic -timeout 360m -parallel 4
=== RUN   TestAccIecEIPResource_basic
=== PAUSE TestAccIecEIPResource_basic
=== CONT  TestAccIecEIPResource_basic
--- PASS: TestAccIecEIPResource_basic (33.19s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       33.227s