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

This patch fixes the following warnings:

drivers/hwtracing/intel_th/sth.c: In function ‘sth_stm_packet’:
drivers/hwtracing/intel_th/sth.c:86:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   reg += 4;
   ~~~~^~~~
drivers/hwtracing/intel_th/sth.c:87:2: note: here
  case STP_PACKET_XSYNC:
  ^~~~
drivers/hwtracing/intel_th/sth.c:88:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   reg += 8;
   ~~~~^~~~
drivers/hwtracing/intel_th/sth.c:89:2: note: here
  case STP_PACKET_TRIG:
  ^~~~

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

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: Alexander Shishkin <alexander.shishkin@linux.intel.com>