Changes

sctp_proxy: add _from_{enb,mme} variants of sctp_send/2

The existing sctp_send/2 is sending data from an eNB towards
the MME.  Sending data in the opposite direction is done in-place.

For the sake of readability, let's add the counterpart of sctp_send/2
and clarify the direction by adding _from_{enb,mme} to function names.

Change-Id: I388137e27ea5000f22e7a5b58d1ffcfd9caa1475