clk: Fix return value check in oxnas_stdclk_probe()

In case of error, the function syscon_node_to_regmap() 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>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Fixes: 0bbd72b4 ("clk: Add Oxford Semiconductor OXNAS Standard Clocks")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>