Ethernet: Rework the length checks

Add a sanity check: packet length need to be >= capture length

(Like a sanity check in print.c, pretty_print_packet() function)

Remove a now useless test:
Because packet length (length) >= capture length (caplen), when
caplen >= ETHER_HDRLEN + switch_tag_len, length cannot
be < ETHER_HDRLEN + switch_tag_len.