net: caif: use correct format specifier

%u is the wrong format specifier for int.
size_t cannot be converted to int without possible
loss of information.

So leave the result as size_t and use %zu as format specifier.

cf. Documentation/printk-formats.txt

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>