mlxsw: spectrum: Expose counter for all 16 TCs

Before MC-aware mode was enabled in commit 7b819530

 ("mlxsw:
spectrum: Configure MC-aware mode on mlxsw ports"), only 8 traffic
classes were used. Under MC-aware regime, however, besides using TCs
0-7 for UC traffic, it additionally uses TCs 8-15 for BUM traffic. It
is therefore desirable to show counters for these TCs as well.

Update ethtool stats pool length, mlxsw_sp_port_get_strings() and
mlxsw_sp_port_get_stats() to include artifacts for all 16 TCs. For
consistency and simplicity, expose tc_no_buffer_discard_uc_tc for BUM
TCs as well, even though it ought to stay at 0 all the time.

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