jtag: rewrite jim_jtag_tap_enabler() as COMMAND_HANDLER

The function is used for commands:
- jtag tapisenabled
- jtag tapenable
- jtag tapdisable

While there, add the missing .help and .usage fields and fix the
incorrect check in jtag_tap_enable() and jtag_tap_disable().

Change-Id: I0e1c9f0b8d9fbad19d09610a97498bec8003c27e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7554
Tested-by: jenkins

src/jtag/tcl.h

@@ -18,9 +18,10 @@
 #ifndef OPENOCD_JTAG_TCL_H
 #define OPENOCD_JTAG_TCL_H
 
+#include <helper/command.h>
+
 int jim_jtag_configure(Jim_Interp *interp, int argc,
 		Jim_Obj * const *argv);
-int jim_jtag_tap_enabler(Jim_Interp *interp, int argc,
-		Jim_Obj * const *argv);
+__COMMAND_HANDLER(handle_jtag_tap_enabler);
 
 #endif /* OPENOCD_JTAG_TCL_H */