firmware: arm_scmi: Fix type error assignment in voltage protocol

firmware: arm_scmi: Fix type error assignment in voltage protocol

Fix incorrect type assignment error reported by sparse as:

drivers/firmware/arm_scmi/voltage.c:159:42: warning: incorrect type in assignment (different base types)
drivers/firmware/arm_scmi/voltage.c:159:42: expected restricted __le32 [usertype] level_index
drivers/firmware/arm_scmi/voltage.c:159:42: got unsigned int [usertype] desc_index

Link: https://lore.kernel.org/r/20211115154043.49284-1-cristian.marussi@arm.com
Fixes: 9f4799c1f2cec ("firmware: arm_scmi: Add voltage domain management protocol support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>