Bluetooth: btintel: Constify static struct regmap_bus

The only usage of regmap_ibt is to (after the regmap_init() macro is
expanded), pass its address to __regmap_init(), which takes a pointer to
const struct regmap_bus as input. Make it const to allow the compiler to
put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>