Changes

rename to umts_cell_id_to_str()

A subsequent patch adds umts_cell_id_to_str_buf(), and before the old
foo_name() pattern spreads further, I'd rather rename it.

Rationale:
- There is a umts_cell_id_from_str() function.
- The foo_name() is an old pattern, we prefer foo_to_str() now.
- Contained within osmo-hnbgw, no API problems.

Change-Id: I3124d1f5e634bc895ec347cb1a9816789fd9ab69