clk: sunxi: signedness bug in sun8i_h3_bus_gates_init()

"index" needs to be signed for the error handling to work.

Fixes: ab6e23a4 ('clk: sunxi: Add H3 clocks support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>