dtls: extract cipher comparison into a own functions
dtls: extract cipher comparison into a own functions

Move the comparison of the used cipher into a inline function. This
makes it easier to deactivate the code when one of these ciphers is not
used at compile time.

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

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