Use OSMO_FD_* instead of deprecated BSC_FD_*

New define is available since libosmocore 1.1.0, and we already require
1.2.0, so no need to update dependenices.
Let's change it to avoid people re-using old BSC_FD_* symbols when
copy-pasting somewhere else.

Change-Id: Iaebd049e383b02204a12f39cc6c932a53d25fd72