Store the TLS version in tls_version instead of major, minor version num
Note: existing application use which accesses the struct member
(using MBEDTLS_PRIVATE) is not compatible on little-endian platforms,
but is compatible on big-endian platforms. For systems supporting
only TLSv1.2, the underlying values are the same (=> 3).
New setter functions are more type-safe,
taking argument as enum mbedtls_ssl_protocol_version:
mbedtls_ssl_conf_max_tls_version()
mbedtls_ssl_conf_min_tls_version()
Signed-off-by: Glenn Strauss <gstrauss@gluelogic.com>