iio:dac:m62332: share scale and offset

This device simply uses its Vcc as reference voltage, so the same scale
applies for all channels. Also offset doesn't appear to be different for
any channel. Represent this by switching these two attributes to
info_mask_shared_by_type.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>