tty: mips_ejtag_fdc: Mark expected switch fall-through

Mark mips_ejtag_fdc_encode() methods switch-case-4 as expecting to
fall through.

This patch fixes the following warning:

drivers/tty/mips_ejtag_fdc.c: In function ‘mips_ejtag_fdc_encode’:
drivers/tty/mips_ejtag_fdc.c:245:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
   word.word &= 0x00ffffff;
   ~~~~~~~~~~^~~~~~~~~~~~~
drivers/tty/mips_ejtag_fdc.c:246:2: note: here
  case 3:
  ^~~~

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Link: https://lore.kernel.org/r/20200306124913.151A68030792@mail.baikalelectronics.ru
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/mips_ejtag_fdc.c

@@ -243,6 +243,7 @@ done:
 		/* Fall back to a 3 byte encoding */
 		word.bytes = 3;
 		word.word &= 0x00ffffff;
+		/* Fall through */
 	case 3:
 		/* 3 byte encoding */
 		word.word |= 0x82000000;