kgdb,x86_64: fix compile warning found with sparse

Fix sparse warning:

arch/x86/kernel/kgdb.c:123:9: warning: switch with no cases

Reported-by: Namhyung Kim <[e-mail]>
Signed-off-by: Jason Wessel <[e-mail]>

arch/x86/kernel/kgdb.c

···
 		memcpy(mem, (void *)regs + dbg_reg_def[regno].offset,
 		       dbg_reg_def[regno].size);
 
-	switch (regno) {
 #ifdef CONFIG_X86_32
+	switch (regno) {
 	case GDB_SS:
 		if (!user_mode_vm(regs))
 			*(unsigned long *)mem = __KERNEL_DS;
···
 	case GDB_FS:
 		*(unsigned long *)mem = 0xFFFF;
 		break;
-#endif
 	}
+#endif
 	return dbg_reg_def[regno].name;
 }