ACPI: CPPC: initialise vaddr pointers to NULL

Properly initialise vaddr pointers in cpc_read() and cpc_write() to
NULL instead of 0.

This fixes the following sparse warnings:

drivers/acpi/cppc_acpi.c:937:31: warning: Using plain integer as NULL pointer
drivers/acpi/cppc_acpi.c:982:31: warning: Using plain integer as NULL pointer
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/cppc_acpi.c

@@ -934,7 +934,7 @@ int __weak cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val)
 static int cpc_read(int cpu, struct cpc_register_resource *reg_res, u64 *val)
 {
 	int ret_val = 0;
-	void __iomem *vaddr = 0;
+	void __iomem *vaddr = NULL;
 	int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpu);
 	struct cpc_reg *reg = &reg_res->cpc_entry.reg;
 
@@ -979,7 +979,7 @@ static int cpc_read(int cpu, struct cpc_register_resource *reg_res, u64 *val)
 static int cpc_write(int cpu, struct cpc_register_resource *reg_res, u64 val)
 {
 	int ret_val = 0;
-	void __iomem *vaddr = 0;
+	void __iomem *vaddr = NULL;
 	int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpu);
 	struct cpc_reg *reg = &reg_res->cpc_entry.reg;