clk: sprd: Check error only for devm_regmap_init_mmio()

The function devm_regmap_init_mmio() wouldn't return NULL pointer for
now, so only need to ensure the return value is not an error code.

Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>