For USRP2, implement auto scaling of TX pipeline such that [-1.0 1.0] input to

For USRP2, implement auto scaling of TX pipeline such that [-1.0 1.0] input to
usrp2.sink_32fc results in full-scale DAC outputs, regardless of interpolation
rate.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@10182 221aa14e-8319-0410-a670-987f0aec2ac5