net: ipc_router: Fix format specifiers in debug statements

%zu is the correct printk format specifier when printing size_t types.

Update printks to use %zu for size_t type variables.

Change-Id: I550b7853aeb1bdaaadf23ecfa9e1383b7f15ad89
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>