mlxsw: spectrum: Fix SPAN egress mirroring buffer size for Spectrum-2

For SPAN egress mirroring buffer size, it is needed to use a different
formula for Spectrum and Spectrum-2. Move the buffer size computation to
ops and implement new formula for Spectrum-2.

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