lib: add default labels and comments to switch statements

According to the Zephyr Coding Guideline all switch statements
shall be well-formed.
Added a default labels to switch-clauses without them.
Added comments to the empty default cases.

Found as a coding guideline violation (MISRA R16.1) by static
coding scanning tool.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>