net: icmpv6: Simplify debug calls

Debugging code snippet inside handle_echo_reply() does not look nice,
just provided inline functions and simplified it.

Change-Id: Idc3a9284153aaa04d0431a9dd705c4f1c51b817d
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>