drivers/jtag: usb_blaster: make command handlers more strict

If user used wrong argument number for some usb_blaster_* commands
then openocd show just warning message.
This commit makes command handler's behaviour more strict and
openocd treats wrong argument number as an error.

In addition we already have 'help' and 'usage' information in
struct command_registration ublast_command_handlers[],
so we can drop redundancy messages in command handlers.

Change-Id: I73b8c75ec60a18e5258a4bdffe972e8a1afc1066
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Reviewed-on: http://openocd.zylin.com/1942
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>