Changes

ipaccess-proxy: Fix wrong cb function passed

It makes no sense that ipaccess-proxy is using the ipaccess_fd_cb()
function from libosmo-abis, since that function expects an e1_input
ipaccess driver to be set up by the app, which ipaccess-proxy never sets
up. Furthermore, even the data pointer being passed doesn't match the
one expected by the callback.

This seems to be a bug introduced 10 years ago in osmo-bsc.git
eb623019382fdbff36fb8a72052732e225aef67e, where the name of the private
function callback used by ipaccess-proxy was changed to
proxy_ipaccess_fd_cb() to avoid colliding with the ipaccess_fd_cb() from
libosmo-abis, but only 1 of the 2 references was changed at the time,
leaving the other one pointing to the wrong function.

Fixes: eb623019382fdbff36fb8a72052732e225aef67e
Change-Id: I1702d9913a462a36e4b78b503a8338ae2ad140b1