clk: aspeed: Fix return value check in aspeed_cc_init()

In case of error, the function of_iomap() returns NULL pointer not
ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Fixes: a2e230c7b2ea ("clk: Add clock driver for ASPEED BMC SoCs")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>