staging:iio:hmc5843: Constify register tables and struct regmap_config

The regmap_access_table and regmap_config structures may be const
because they are not modified by the driver and regmap_init() accepts
pointer to const.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>