net: link_local: Fix netmask endianness bug

The network mask must be stored in network order when in a
'struct in_addr'.

This fix removes the "gatewayip needed but not set" message on the
console when using a link-local IP setup.

Signed-off-by: Alexandre Messier <amessier@tycoint.com>
Signed-off-by: Sylvain Lemieux <slemieux@tycoint.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>