iio: 104-quad-8: Fix index control configuration

The LS7266R1 requires bits 5 & 6 to be high in order to select the Index
Control Register. This patch fixes a typo that incorrectly selects the
Input/Output Control Register where the Index Control Register was
desired.

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>