treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/drivers/ssb/scan.c b/drivers/ssb/scan.c
index b97a5c3..f49ab1a 100644
--- a/drivers/ssb/scan.c
+++ b/drivers/ssb/scan.c
@@ -228,7 +228,7 @@ static void __iomem *ssb_ioremap(struct ssb_bus *bus,
 	switch (bus->bustype) {
 	case SSB_BUSTYPE_SSB:
 		/* Only map the first core for now. */
-		/* fallthrough... */
+		fallthrough;
 	case SSB_BUSTYPE_PCMCIA:
 		mmio = ioremap(baseaddr, SSB_CORE_SIZE);
 		break;