dtls: increase buffer size for ecdsa
dtls: increase buffer size for ecdsa

The server Hello, Certificate and server key exchange flight is needs
more space.

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

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