[zircon][rtc] Fixes and some tests for rtclib

The new fallback driver needs to return sizeof(struct)
rather than ZX_OK. I was misled by the DDKTL.

Doing some cleanup and noticed no tests. Also made it
so there are less magic numbers.

TEST= unittest included. Also manual test on Pixelbook.

Change-Id: I5531d3888bae601179b4fcdd0949a3fdcdc6768f