iio: qcom-vadc-common: Correct order of adc scaling functions

Correct the order of ADC scaling functions in the enum to align
with the existing defined values used optionally to set scaling
function from device tree.

Change-Id: I21754559f417c70ea5f8f43e2ade39d96e931f0d
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>