iio: adc: xilinx: Fix VREFP scale

The scaling factor for VREFP is 3.0/4096, not 1.0/4096; fix this to get
correct readings.

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>