clk: sunxi: Fix return value check in sun8i_a23_mbus_setup()

In case of error, the function of_io_request_and_map() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>