tipc: make media address offset a common define

With the exception of infiniband media which does not use media
offsets, the media address is always located at offset 4 in the
media info field as defined by the protocol, so we move the
definition to the generic bearer.h

Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>