Follow the model of `MBEDTLS_SSL_CONF_ENDPOINT`. This saves a small
amount - most of the saving was already acheived via`
MBEDTLS_SSL_TRANSPORT_IS_TLS` but we can scrape out a little more by
totally eliminating `ssl->conf->transport` references.
Signed-off-by: Kevin Bracey <kevin.bracey@arm.com>