firmware: ti_sci: Use %zu for size_t print format

mbox_msg->len is of type size_t and %d is incorrect format. Instead
use %zu for handling size_t correctly.

Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>