net: uip: Return correct packet discard status to caller

Fixed the compiler warning about not returning correct value to
caller. Now we return correct value if neighbor solicitation was
sent properly.

Change-Id: Ie28a2ee417bcdf3eb9b24ab91a49c688992e7420
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>