sh: kernel: disassemble: Mark expected switch fall-throughs

Remove logically dead code and mark switch cases where we are expecting
to fall through.

Fix the following warnings (Building: defconfig sh):

arch/sh/kernel/disassemble.c:478:8: warning: this statement may fall
through [-Wimplicit-fallthrough=]
arch/sh/kernel/disassemble.c:487:8: warning: this statement may fall
through [-Wimplicit-fallthrough=]
arch/sh/kernel/disassemble.c:496:8: warning: this statement may fall
through [-Wimplicit-fallthrough=]
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

arch/sh/kernel/disassemble.c

@@ -475,8 +475,6 @@ static void print_sh_insn(u32 memaddr, u16 insn)
 				printk("dbr");
 				break;
 			case FD_REG_N:
-				if (0)
-					goto d_reg_n;
 			case F_REG_N:
 				printk("fr%d", rn);
 				break;
@@ -488,7 +486,7 @@ static void print_sh_insn(u32 memaddr, u16 insn)
 					printk("xd%d", rn & ~1);
 					break;
 				}
-			d_reg_n:
+				/* else, fall through */
 			case D_REG_N:
 				printk("dr%d", rn);
 				break;
@@ -497,6 +495,7 @@ static void print_sh_insn(u32 memaddr, u16 insn)
 					printk("xd%d", rm & ~1);
 					break;
 				}
+				/* else, fall through */
 			case D_REG_M:
 				printk("dr%d", rm);
 				break;