jtag: rewrite jim_jtag_configure() as COMMAND_HANDLER

The function is used for commands:
- jtag configure
- jtag cget

While there, add the missing .usage field.

Change-Id: I97ddc4898259ddb7fd2d057a997f33a6f4b0e2a8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8058
Tested-by: jenkins

src/target/adi_v5_dapdirect.c

@@ -118,7 +118,8 @@ static const struct command_registration dapdirect_jtag_subcommand_handlers[] =
 	{
 		.name = "cget",
 		.mode = COMMAND_EXEC,
-		.jim_handler = jim_jtag_configure,
+		.handler = handle_jtag_configure,
+		.usage = "",
 	},
 	{
 		.name = "names",