jtag: usb_blaster: Add missing 'default' to switch statement

jtag: usb_blaster: Add missing 'default' to switch statement

If a new JTAG command is added, then GCC will complain that
enumeration value not handled in switch. This is the only driver not
to have a default case, so add it.

Change-Id: Icb838087bb7525d057a911bd256300e256da1668
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://openocd.zylin.com/5333
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>