firmware: arm_scmi: Use NULL instead of integer 0 for rate pointer

Kbuild test robot reports the following sparse warning:

drivers/firmware/arm_scmi/clock.c:142:21:
	sparse: Using plain integer as NULL pointer

Use NULL pointer instead of integer 0 for rate pointer and fix the
warning.

Link: https://lore.kernel.org/r/20200717140405.17905-1-sudeep.holla@arm.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

drivers/firmware/arm_scmi/clock.c

@@ -139,7 +139,7 @@ static int
 scmi_clock_describe_rates_get(const struct scmi_handle *handle, u32 clk_id,
 			      struct scmi_clock_info *clk)
 {
-	u64 *rate = 0;
+	u64 *rate = NULL;
 	int ret, cnt;
 	bool rate_discrete = false;
 	u32 tot_rate_cnt = 0, rates_flag;