Fix bug in OLM during spanloss computation

Fix bug in OLM during spanloss computation

Prevent from getting NPE when OTS interface terminating the
roadm-to-roadm link under spanloss determination does not exist in the
device.
- remove invocation of getOtsInterfaceName method on an OtsPmHolder null
  object, at the origin of the NPE thrown.
  This check is superfluous since OtsPmHolder.getOtsInterfaceName() can
  not be null if OtsPmHolder is not null.
- fix typo on OtsPmHolder variable names
- adapt LOG messages consequently

JIRA: TRNSPRTPCE-172
Signed-off-by: Gilles Thouenon <gilles.thouenon@orange.com>
Change-Id: Idec6fac25a33c7748ad4996e01211e04667b4fd5