svf: Only read TDO back from the device if we actually need to look at the bits.

svf: Only read TDO back from the device if we actually need to look at the bits.

This results in a 90% speedup on USB-Blaster, which serializes repeated
TDI input against TDO readback; program time on an 5CGXFC5C6F27 part was
dropped from 2m30s to 9s.

Signed-off-by: Joshua Wise <joshua@joshuawise.com>
Change-Id: I92d5a8b800492283d619328549235b610528c338
Reviewed-on: http://openocd.zylin.com/2145
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>