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

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>