hwmon: (ads7828) Enable internal reference

On ads7828 the internal reference defaults to off upon power up. When
using internal reference, it needs to be turned on and the voltage needs
to settle before normal conversion cycle can be started. Hence perform a
dummy read in the probe to enable the internal reference allowing the
voltage to settle before performing a normal read.

Without this fix, the first read from the ADC when using internal
reference always returns incorrect data.

Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>