powerpc: annotate implicit fall throughs

There is a plan to build the kernel with -Wimplicit-fallthrough and these
places in the code produced warnings, but because we build arch/powerpc
with -Werror, they became errors.  Fix them up.

This patch produces no change in behaviour, but should be reviewed in
case these are actually bugs not intentional fallthoughs.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powermac/feature.c

@@ -1471,6 +1471,7 @@ static long g5_i2s_enable(struct device_node *node, long param, long value)
 	case 2:
 		if (macio->type == macio_shasta)
 			break;
+		/* fall through */
 	default:
 		return -ENODEV;
 	}