net: Fix alignment error with net_ipaddr_copy()

As struct sockaddr have now alignment of 4 bytes, net_ipaddr_copy()
gives the following error if used for sockaddr:

  error: alignment 1 of ‘struct <anonymous>’ is less than 4
  [-Werror=packed-not-aligned]

Just use memcpy() instead, net_ipaddr_copy() was intended to use with IP
addresses, not socket related structs.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>