intel_scu_ipc: fix size field for intel_scu_ipc_command

Size for PMIC read/write command is byte, while it is DWORD for other
IPC commands.

Signed-off-by: Hong Liu <hong.liu@intel.com>
Signed-off-by: ALan Cox <alan@linux.intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>