hwmon: (emc1403) Add support for conversion interval configuration

The chips supported by the emc1403 driver support configurable
conversion rates. Add support for it.

Cc: Lars Petter Mostad <lars.petter.mostad@appear.net>
Tested-by: Lars Petter Mostad <lars.petter.mostad@appear.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>