pcapparse: fix SLL parsing

pcapparse: fix SLL parsing

The current code is not checking for ethernet type, as it's supposed to,
but link layer device type and it's hard-coded to only accept dumps from
ethernet (ARPHRD_ETHER; 1). We don't care where the dump was fetched
from (wlan, 3G, etc.)

What we care about is the that the ethernet type is IP (ETHERNET_IP;
0x800), which is clearly field 14:

http://www.tcpdump.org/pcap3_man.html

And do a bit of cleanup.

Signed-off-by: Felipe Contreras <felipe.contreras@nokia.com>