net: ipv6: Prefix length was incorrectly set

The IPv6 prefix address length was incorrectly set in net_if
after RA was received from the network. The incorrect "len"
variable was used instead of correct "prefix_len" in the prefix
info struct.

Change-Id: Ifeaf150b9960414d2ad200053a3bd5290dc8d365
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>