hwmon: epm_adc: Fix incorrect SPI writes

Incorrect spi write transactions and incorrect readings
are seen. This is because of using incorrect SPI slave
address while initiating the SPI writes to the ADS ADC.
Also fix the incorrect scaling when converting the raw ADC
code to the appropriate physical result.

Change-Id: I0e16f210fa07e1ce1128f39b0f4e1d325ea7e7ef
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>