Add support for decoding Broadcom Ethernet switches tags

Add support for decoding Broadcom Ethernet switches tags

This adds support for decoding the Broadcom Ethernet switches tags which
are are 4byte in length and are located between the Ethernet MAC SA and
the Type/Length field (DSA_TAG_BRCM) as well as the pre-pended Broadcom
tag (DSA_TAG_BRCM_PREPEND) which are located in front of the standard
Ethernet header.

This makes use of the recently introduced ether_print_hdr_len() to allow
specifying the non-standard Ethernet header length of 12 + 4 + 2 bytes.