iio: 104-quad-8: Fix off-by-one errors when addressing IOR

The Input/Output Control register (IOR) is offset by 1 from the
respective channel data register. This patch fixes off-by-one errors
when attempting to write to a channel IOR where the base address was not
properly offset.

Fixes: 28e5d3bb ("iio: 104-quad-8: Add IIO support for the ACCES 104-QUAD-8")
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>