iio: light: make a couple of config structures static

Structures st_uvis25_i2c_regmap_config and st_uvis25_spi_regmap_config are
local to the source and do not need to be in global scope, so make them
both static.

Cleans up sparse warnings:
warning: symbol 'st_uvis25_i2c_regmap_config' was not declared. Should
it be static?
warning: symbol 'st_uvis25_spi_regmap_config' was not declared. Should
it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>