iio: 104-quad-8: Fix off-by-one error when addressing flag register

The flag register is offset by 1 from the respective channel data
register. This patch fixes an off-by-one error when attempting to read a
channel flag register 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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>