mfd: ti_am335x_tscadc: Fix idle timeout value

The old timeout value was based on the assumption that the minimum values are
used for the open and sample delay and no averaging is done. In fact the ADC
and touchscreen driver both use an open delay of 152 cycles and averaging over
16 samples. This patch adjusts the timeout value accordingly

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>