intel_pmc_ipc: Fix compiler casting warnings

Avoid casting variables to different sizes due to different
compilers and settings.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: qipeng.zha <qipeng.zha@intel.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>