scsi: st: Fix fall-through warning for Clang

Fix the following fallthrough warning:

drivers/scsi/st.c:3826:2: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
drivers/scsi/st.c:3826:2: note: insert 'break;' to avoid fall-through

This helps with the ongoing efforts to globally enable
-Wimplicit-fallthrough for Clang.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/linux-mm/202109070707.4pPukuYB-lkp@intel.com/


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>