target: use 'unsigned int' for smp group

Change the type to 'struct target::smp' and to the initialization
variable 'smp_group'.

Change-Id: I5f5a30a796aaf4e0014a38e81abdf4fb4afbdf48
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8478
Reviewed-by: zapb <dev@zapb.de>
Tested-by: jenkins

src/target/target.c

@@ -6007,7 +6007,7 @@ static int get_target_with_common_rtos_type(struct command_invocation *cmd,
 
 COMMAND_HANDLER(handle_target_smp)
 {
-	static int smp_group = 1;
+	static unsigned int smp_group = 1;
 
 	if (CMD_ARGC == 0) {
 		LOG_DEBUG("Empty SMP target");