thermal: generic-adc: Fix adc to temp interpolation

First correct the edge case to return the last element if we're
outside the range, rather than at the last element, so that
interpolation is not omitted for points between the two last entries in
the table.

Then correct the formula to perform linear interpolation based the two
points surrounding the read ADC value. The indices for temp are kept as
"hi" and "lo" to pair with the adc indices, but there's no requirement
that the temperature is provided in descendent order. mult_frac() is
used to prevent issues with overflowing the int.

Cc: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>