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

The devm_kzalloc() function returns NULL on error, not error pointers.
Update the check to match.

Fixes: ebac87cd ("clk: mmp: Add Marvell PXA1908 MPMU driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/5b3b963d-ecae-4819-be47-d82e8a58e64b@stanley.mountain


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