jlink: improve USB read during jlink_tap_execute

Previously, when doing EMU_CMD_HW_JTAG3 commands we would do two reads,
one to read the data, and one to read the result. However, we can just
do a single larger read instead.

The motivation for this change is a weird problem. If I run the Segger
software before running OpenOCD, then the first read always fails:
Error: usb_bulk_read failed (requested=1, result=0)
Error: jlink_tap_execute, wrong result -107 (expected 1)

Sniffing the USB traffic shows that the J-Link is returning an
overflow error, meaning it is expecting to return the full result
in a single read.

Change-Id: I75e020d3b3732c9a74ee3d31838fdf17a7fac24c
Signed-off-by: Philip Craig <phil@blackmoth.com.au>
Reviewed-on: http://openocd.zylin.com/1447


Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>