scsi: aha1740: Fix fall-through warnings for Clang

scsi: aha1740: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning by
explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/r/e9fc10eb7d843e6f31e50400d428bd7a217684ac.1605896060.git.gustavoars@kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>