modules: mbedtls: Fix buffer size configs for 3.0

Mbed TLS 3.0 changes the configuration defines used to determine the
buffer size used for TLS.  We were still setting the old one, which was
causing Mbed TLS to revert back to the large default buffer size.  Set
both the in and out buffer sizes to match the config setting.  A future
improvement may be to separate this into two configurations within the
Zephyr config.

Signed-off-by: David Brown <david.brown@linaro.org>