twl4030-madc: Convert ioctl to unlocked

Use unlocked ioctl instead of taking the BKL for an operation that can
take some milliseconds. The driver mutex is now taken before the active
status check, so that the concurrent users of this ioctl won't start
seeing EBUSY (previously guaranteed by the BKL).

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