amd-xgbe: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

In file included from drivers/net/ethernet/amd/xgbe/xgbe-drv.c:129:
drivers/net/ethernet/amd/xgbe/xgbe-drv.c: In function ‘xgbe_set_hwtstamp_settings’:
drivers/net/ethernet/amd/xgbe/xgbe-common.h:1392:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  (_var) |= (((_val) & ((0x1 << (_width)) - 1)) << (_index)); \
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-common.h:1419:2: note: in expansion of macro ‘SET_BITS’
  SET_BITS((_var),      \
  ^~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-drv.c:1614:3: note: in expansion of macro ‘XGMAC_SET_BITS’
   XGMAC_SET_BITS(mac_tscr, MAC_TSCR, TSVER2ENA, 1);
   ^~~~~~~~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-drv.c:1616:2: note: here
  case HWTSTAMP_FILTER_PTP_V1_L4_EVENT:
  ^~~~
In file included from drivers/net/ethernet/amd/xgbe/xgbe-drv.c:129:
drivers/net/ethernet/amd/xgbe/xgbe-common.h:1392:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  (_var) |= (((_val) & ((0x1 << (_width)) - 1)) << (_index)); \
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-common.h:1419:2: note: in expansion of macro ‘SET_BITS’
  SET_BITS((_var),      \
  ^~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-drv.c:1625:3: note: in expansion of macro ‘XGMAC_SET_BITS’
   XGMAC_SET_BITS(mac_tscr, MAC_TSCR, TSVER2ENA, 1);
   ^~~~~~~~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-drv.c:1627:2: note: here
  case HWTSTAMP_FILTER_PTP_V1_L4_SYNC:
  ^~~~
In file included from drivers/net/ethernet/amd/xgbe/xgbe-drv.c:129:
drivers/net/ethernet/amd/xgbe/xgbe-common.h:1392:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  (_var) |= (((_val) & ((0x1 << (_width)) - 1)) << (_index)); \
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-common.h:1419:2: note: in expansion of macro ‘SET_BITS’
  SET_BITS((_var),      \
  ^~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-drv.c:1636:3: note: in expansion of macro ‘XGMAC_SET_BITS’
   XGMAC_SET_BITS(mac_tscr, MAC_TSCR, TSVER2ENA, 1);
   ^~~~~~~~~~~~~~
drivers/net/ethernet/amd/xgbe/xgbe-drv.c:1638:2: note: here
  case HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comments are modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>