iio: mxs-lradc: fix divider

All channels' single measurement are happening on CH 0. So enabling / disabling
the divider once is not enough, because it has impact on all channels.

Set only a flag, then check this on each measurement, and enable / disable the
divider as required.

Signed-off-by: Robert Hodaszi <robert.hodaszi@digi.com>
Cc: Stable@vger.kernel.org
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>