hwmon: epm_adc: Update the physical result

The scaling function multiplies the ADC code with
a high scale factor to avoid rouding accuracy errors.
This results in a 32 bit integer not being sufficient
to store the result. Therefore use a 64 bit integer
type to process the result.

Change-Id: Iee5ec105d66012efef9e9674de2e8aa0c886d31f
Signed-off-by: Yan He <yanhe@codeaurora.org>