Don't use the printf format %zd
Don't use the printf format %zd

We target C89 libc, so don't use %zd or %zu. Just use %u, and make
slot numbers `unsigned` for simplicity.

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls