net: Make sure IP layer events have fully differentiated layer code

Layer code should be made so that masking them don't overlap with
another one in net_event code logic. Earlier it was possible to get
an IP addr event for IPv6 even though code would be only listening
for IPv4 one.

Reported-by: Xiaorui Hu <xiaorui.hu@linaro.org>

Change-Id: I8341a91d55556033dd228f68f8ca64e196f52bec
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>