bus: sunxi-rsb: Always check register address validity

The register address was already validated for read operations in
regmap_sunxi_rsb_reg_read before being truncated to a u8. Write operations
have the same set of possible addresses, and the address is being truncated
from u32 to u8 here as well, so the same check is needed.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Fixes: d787dcdb9c8f ("bus: sunxi-rsb: Add driver for Allwinner Reduced Serial Bus")
Reviewed-by: Chen-Yu Tsai <wens@kernel.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://patch.msgid.link/20260301144939.1832806-1-andrej.skvortzov@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>