mlxsw: spectrum_buffers: Split TC_COUNT into ingress and egress

Current code assumes that ingress and egress has the same number of
traffic classes. Since the introduction of MC-aware mode that assumption
hasn't held anymore, and there have been 16 TCs on the egress as opposed
to 8 on ingress.

Break the assumption of symmetry by splitting the artifacts related to
shared-buffer TC counting to ingress and egress parts.

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