net: ethernet: Fixes for ethernet stats reporting.

The following fixes were applied:
 - Multicast Rx packets stats were not recorded due not parsing the
ethernet header. The function that tried to parse the ethernet header
was parsing the ethernet packet beyond the ethernet header.
 - Added a new stats for unknown protocol which gets updated when the
ethernet layer encounters an unknown ethernet packet type.

Fixes #53994

Signed-off-by: Chamira Perera <chamira.perera@audinate.com>