Process the flags even if parsing the full radiotap header fails.

If we were able to get the flags, we should process them, so we know
whether there's Atheros padding after the 802.11 header or if there's an
FCS at the end of the packet, and properly process them; if we weren't
able to get the flags, they're 0, so we're not doing anything
differently, and that's the best we can do.