UmTRX: Manually set Tx gain stages for the best signal quality.

New UHD versions support split configuration of Tx gain stages.
We utilize this to set the gain configuration, optimal for
the Tx signal quality. From our measurements, VGA1 must be
18dB plus-minus one and VGA2 is the best when 23dB or lower.

Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
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