lmb: Fix lmb_add_region_flags() return codes and testing

lmb: Fix lmb_add_region_flags() return codes and testing

The function description says this should return 0 or -1 on failures.
When regions coalesce though this returns the number of coalescedregions
which is confusing and requires special handling of the return code.
On top of that no one is using the number of coalesced regions.

So let's just return 0 on success and adjust our selftests accordingly

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>