twl4030-madc: Let the operation complete faster

No need to wait before the first read, as the operation is likely
completed already. For timeout 50 milliseconds is an overkill, poll the
register for the duration of 5 milliseconds at maximum.

Also, clear the active flag in case of timeout.

Signed-off-by: Aaro Koskinen <Aaro.Koskinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>