dtls: only advertise PSK cipher if supported
dtls: only advertise PSK cipher if supported

Only advertise and support the PSK cipher if the corresponding callback
was implemented and fail otherwise.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

yunus/TinyDTLS-WebID,spark/tinydtls,thecodemaiden/tinydtls-fork,jvermillard/tinydtls,OlegHahm/tinydtls,OlegHahm/tinydtls,nmacs/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,yunus/TinyDTLS-WebID,spark/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,jvermillard/tinydtls,fvdnabee/tinydtls,thecodemaiden/tinydtls-fork,OlegHahm/tinydtls,thecodemaiden/tinydtls-fork,jvermillard/tinydtls,nmacs/tinydtls,spark/tinydtls,yunus/TinyDTLS-WebID