net: use UNALIGNED_GET/PUT to access IP address

Use UNALIGNED_GET and UNALIGNED_PUT throughout the networking stack to
access fields from the IP address structure. These structures can be
mapped directly to buffers and the macros are required for correct
unaligned memory access.

Jira: ZEP-2012

Change-Id: I55f9da7b143a22fa869d5d215c661de988cd9b91
Signed-off-by: Bogdan Davidoaia <bogdan.davidoaia@linaro.org>