MIPS: Annotate fall-through in Cavium Octeon code

There's an intentional switch case fall-through in Cavium Octeon USB
code, which triggers compile errors with -Wimplicit-fallthrough due to
-Werror being enabled for arch/mips.

This can be encountered when building cavium_octeon_defconfig.

Fix the build issue by annotating the intentional fall-through.
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org

arch/mips/cavium-octeon/octeon-usb.c

@@ -398,6 +398,7 @@ static int dwc3_octeon_clocks_start(struct device *dev, u64 base)
 	default:
 		dev_err(dev, "Invalid ref_clk %u, using 100000000 instead\n",
 			clock_rate);
+		/* fall through */
 	case 100000000:
 		mpll_mul = 0x19;
 		if (ref_clk_sel < 2)