net: if: ipv6_prefix_find() correctly tests if the prefix is used

ipv6_prefix_find() wrongly tests if the unicast address is in use
instead of the prefix. This has two implications:

- The function can return an expired prefix to net_if_ipv6_prefix_add(),
  which will do nothing more to enable it (since it assumes that it is
  already enabled). As a result, the prefix will not be used by the
  rest of the stack due to prefix->is_used being false.

- ipv6_prefix_find() loops using a bound of NET_IF_MAX_IPV6_PREFIX, but
  the size of the unicast[] array is defined by NET_IF_MAX_IPV6_ADDR.
  This could lead to an out-of-bound access if NET_IF_MAX_IPV6_ADDR is
  smaller than NET_IF_MAX_IPV6_PREFIX.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>