clk: npcm7xx: fix return value check in npcm7xx_clk_init()

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

Fixes: fcfd1436 ("clk: npcm7xx: add clock controller")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>