sparc64: fix fall-through annotation

Replace "fallthru" with a proper "fall through" annotation.

This fix is part of the ongoing efforts to enabling
-Wimplicit-fallthrough

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/arch/sparc/kernel/kgdb_64.c b/arch/sparc/kernel/kgdb_64.c
index d5f7dc6..a68bbddbd 100644
--- a/arch/sparc/kernel/kgdb_64.c
+++ b/arch/sparc/kernel/kgdb_64.c
@@ -148,7 +148,7 @@
 			linux_regs->tpc = addr;
 			linux_regs->tnpc = addr + 4;
 		}
-		/* fallthru */
+		/* fall through */
 
 	case 'D':
 	case 'k':