clk: sprd: Use IS_ERR() to validate the return value of syscon_regmap_lookup_by_phandle()

The syscon_regmap_lookup_by_phandle() will never return NULL, thus use
IS_ERR() to validate the return value instead of IS_ERR_OR_NULL().

Fixes: d41f59fd

 ("clk: sprd: Add common infrastructure")
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Link: https://lkml.kernel.org/r/1995139bee5248ff3e9d46dc715968f212cfc4cc.1570520268.git.baolin.wang@linaro.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>