af-packet: fix error handling

Only exit from synchronization loop on poll error and not in case
of a timeout.
This fixes the suppress the useless error message:
```
[9496] 11/4/2014 -- 12:02:51 - (source-af-packet.c:1071) <Warning> (AFPSynchronizeStart) -- [ERRCODE: SC_ERR_AFP_READ(191)] - poll failed with retval 0
```
PR builds:
- PR build: https://buildbot.suricata-ids.org/builders/regit/builds/128
- PR pcaps: https://buildbot.suricata-ids.org/builders/regit-pcap/builds/67