drivers: serial: serial_test.c: Patch 64-bit incompat

The serial_test.c driver uses size_t to store read_size,
which becomes a 64-bit type when built for 64-bit
architectures. This is incompatible with the print
format %d which is 32-bit. Updated to %zd

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>