hwmon: (ads1015) Fix build warning

The following build warning is seen in some configurations.

drivers/hwmon/ads1015.c: In function 'show_in':
drivers/hwmon/ads1015.c:129: warning: 'in' may be used uninitialized in this function

Fix by separating the register read function from the code converting the result
into mV.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: Dirk Eibach <eibach@gdsys.de>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>