s390/3215: add switch fall through comment for -Wimplicit-fallthrough

Silence the following warning when built with -Wimplicit-fallthrough=3
enabled by default since 5.3-rc2:
drivers/s390/char/con3215.c: In function 'raw3215_irq':
drivers/s390/char/con3215.c:399:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
  399 |   if (dstat == 0x08)
      |      ^
drivers/s390/char/con3215.c:401:2: note: here
  401 |  case 0x04:
      |  ^~~~

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>