Fix buffer overflow in versaloon interface

The USB buffer will need space for both TMS and TDI buffers.
Each holds tap_buffer_size bytes maximum, so tap_buffer_size must be
smaller than half of usb buf_size.

Change-Id: Id8f39936a894cbd98deb89eec5a859aef1e2b783
Signed-off-by: Joerg Fischer <turboj@gmx.de>
Reviewed-on: http://openocd.zylin.com/1136
Tested-by: jenkins
Reviewed-by: simon qian <simonqian.openocd@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>