mlxsw: core: Parse TLVs' offsets of incoming EMADs

Until now the code assumes a fixed structure which makes it difficult to
support EMADs with and without new TLVs.

Make it more generic by parsing the TLVs when the EMADs are received and
store the offset to the different TLVs in the control block. Using these
offsets to extract information from the EMADs without relying on a specific
structure.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>