bus: sunxi-rsb: unlock on error in sunxi_rsb_read()

Don't forget to unlock before returning an error code.

Fixes: d787dcdb ('bus: sunxi-rsb: Add driver for Allwinner Reduced Serial Bus')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>