ssl_srv.c: Mark ETM as disabled if cipher is not CBC
ssl_srv.c: Mark ETM as disabled if cipher is not CBC

Encrypt-Then-Mac (ETM) is supported in Mbed TLS TLS
1.2 server only for the CBC cipher mode thus make it
clear in the SSL context.

The previous code was ok as long as the check of
the ETM status was done only in the case of the CBC
cipher mode but fragile as #5573 revealed.

Signed-off-by: Ronald Cron <b35ff5b7ca26c4ae9b66b92f2cf34224c8e26b08@arm.com>

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