log: net: Fix build warning with IPv6 disabled

Building the net logger backend with IPv4 only gives the following
warning:

log_backend_net.c:116:31: warning: array subscript 'struct sockaddr[0]'
is partly outside array bounds of 'struct sockaddr_in[1]'
[-Warray-bounds] local_addr->sa_family = server_addr.sa_family;

hence assign the address family directly to sockaddr_in/6 structs.

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