mlxsw: spectrum_ptp: Fix build warnings

Cited commit extended the enums 'hwtstamp_tx_types' and
'hwtstamp_rx_filters' with values that were not accounted for in the
switch statements, resulting in the build warnings below.

Fix by adding a default case.

drivers/net/ethernet/mellanox/mlxsw/spectrum_ptp.c: In function ‘mlxsw_sp_ptp_get_message_types’:
drivers/net/ethernet/mellanox/mlxsw/spectrum_ptp.c:915:2: warning: enumeration value ‘__HWTSTAMP_TX_CNT’ not handled in switch [-Wswitch]
  915 |  switch (tx_type) {
      |  ^~~~~~
drivers/net/ethernet/mellanox/mlxsw/spectrum_ptp.c:927:2: warning: enumeration value ‘__HWTSTAMP_FILTER_CNT’ not handled in switch [-Wswitch]
  927 |  switch (rx_filter) {
      |  ^~~~~~

Fixes: f76510b4

 ("ethtool: add timestamping related string sets")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>