native_posix: Ensure printf fed type matches format specifier

In Zephyr we have the rule of "do not use stdint defined types",
but unfortunately that leaves us with no properly defined
format specifiers for printing values out.
So cast to stdint types in this expression to actually be able to
do so.

Fixes Coverity CID: 205798
Fixes Coverity CID: 205825

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>