iio:dac:m62332: shutdown on remove

The regulator framework requests to balance regulator_enable() calls with
regulator_disable() calls. To meet this requirement, set channels to 0 on
remove, which implies a regulator_disable() call in case that channel was
enabled.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>