net: use UNALIGNED_ACCESS when accessing s_addr on received packets.

The offset of the IP header in a received packet depends on the L2
header size.  For Ethernet this is 14 bytes which puts the u32 IPv4
addresses on a non-u32 byte boundary.  This causes chips that don't
support unaligned access (like the Cortex-M0) to fault.

The fixes in this patch are enough to ping the board and run the
http_server sample.

Signed-off-by: Michael Hope <mlhx@google.com>