scsi: st: Add missing break in switch statement in st_ioctl()

Clang + -Wimplicit-fallthrough warns:

drivers/scsi/st.c:3831:2: warning: unannotated fall-through between
switch labels [-Wimplicit-fallthrough]
        default:
        ^
drivers/scsi/st.c:3831:2: note: insert 'break;' to avoid fall-through
        default:
        ^
        break;
1 warning generated.

Clang's -Wimplicit-fallthrough is a little bit more pedantic than GCC's,
requiring every case block to end in break, return, or fallthrough, rather
than allowing implicit fallthroughs to cases that just contain break or
return. Add a break so that there is no more warning, as has been done all
over the tree already.

Link: https://lore.kernel.org/r/20210817235531.172995-1-nathan@kernel.org
Fixes: 2e27f576

 ("scsi: scsi_ioctl: Call scsi_cmd_ioctl() from scsi_ioctl()")
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>