DoAsn1Key now fails when WOLFSSH_NO_RSA is defined

[gojimmypi]

Updates ssh.c to fix compile-time error when RSA is disabled.

Without this fix, this line in DoAsn1Key():

        ret = wc_RsaFlattenPublicKey(&key->ks.rsa.key, (byte*)&e, &eSz, n, &nSz);

fails to compile as there's no .rsa.key member property:

Severity Description                                   Project                File                                         Line
Error    'union <anonymous>' has no member named 'rsa' wolfssl_IDF_v5.2_ESP32 C:\workspace\wolfssh-master-update\src\ssh.c 1751

With this PR, the code compiles and exits IdentifyAsn1Key() with error code WS_RSA_E and a message when debugging is enabled.