at86rf230: Constify struct regmap_config

The regmap_config struct may be const because it is not modified by the
driver and regmap_init() accepts pointer to const.
Signed-off-by: NKrzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ieee802154/at86rf230.c

@@ -427,7 +427,7 @@ at86rf230_reg_precious(struct device *dev, unsigned int reg)
 	}
 }
 
-static struct regmap_config at86rf230_regmap_spi_config = {
+static const struct regmap_config at86rf230_regmap_spi_config = {
 	.reg_bits = 8,
 	.val_bits = 8,
 	.write_flag_mask = CMD_REG | CMD_WRITE,