EAP-TTLS: Fixed implicit challenge derivation to use correct output length

EAP-TTLS: Fixed implicit challenge derivation to use correct output length

The previous version was deriving unnecessarily large amount of output data
for the implicit challenge. With the TLS PRF, this was not causing any
problems because the output length is not explicitly bound into the
derivation. Anyway, it is better to use the correct output length should
the PRF be ever changed to a one that uses the output length to change the
beginning of the output data.