iio: potentiometer: mcp4531: use pointer to access model parameters

Use const pointer to element from model configuration array rather then array
index, as it will not change anyway.

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>