clk: mmp: pxa1908-apbcp: Fix a NULL vs IS_ERR() check

The devm_kzalloc() function doesn't return error pointers, it returns
NULL on error.  Update the check to match.

Fixes: a89233db ("clk: mmp: Add Marvell PXA1908 APBCP driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/6155067d-aed5-4799-9e14-6dff7be1cb3a@stanley.mountain


Acked-by: Duje Mihanović <duje.mihanovic@skole.hr>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>