[sonic-swss]: update sonic-swss submodule

* 8f52e29 2018-07-25 | Postpone intf task after buffer configuration applied on the port. (#539) (HEAD, origin/201803) [Jipan Yang]
* 2d2fdaa 2018-07-26 | [portsorch]: Refix: Don't print error when alias equal to PortConfigDone (#548) [Qi Luo]
* 87d34db 2018-07-12 | [MirrorOrch]: Fix bug when NULL is returned from the getenv function (#537) [Shuotian Cheng]