scsi: aic7xxx: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding multiple break statements instead of just
letting the code fall through to the next case, and by adding fallthrough
statements in places where the code is intended to fall through, and
finally by replacing /* FALLTHROUGH */ comments with the new pseudo-keyword
macro fallthrough.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/r/1a7cd2f77623e6ab46bbec0b6103b18491419206.1605896059.git.gustavoars@kernel.org


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>