dev/portal: add checks on oflag

It does not make sense to open portal without a flag. One should open a
portal with RT_DEVICE_OFLAG_RDWR in most cases.
dev/portal: fix bug in portal callback forwarding

When forwarding rx/tx callback from the underlaying device(pipe), the
"dev" argument should be the portal. So the portal callback could think
itself as called.
Merge pull request #146 from grissiom/rtt-next

Some fix for the portal device