ipa: fix missing set of write_cb for IPA client connection
ipa: fix missing set of write_cb for IPA client connection

Now the BTS and BSC tests work again.

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis,telenoobie/libosmo-abis,telenoobie/libosmo-abis