char: msm_smd_pkt: Use correct printk format specifier for size_t

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

Update printks to use %zu for size_t type variables.

Change-Id: Iac5d30ee4873c0e660a5166068d17ef4c459df0e
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>