secchan: Disallow port numbers not supported by datapath.

secchan: Disallow port numbers not supported by datapath.

The OpenFlow protocol supports port numbers from 0 to 0xff00.
The datapath protocol supports port numbers from 0 to 0x100.

The datapath will reject port numbers in the range 0x100 to 0xff00, but the
secchan was not screening these out.  Thus, attempting to add a flow or
send a packet to one of these ports would result in a kernel EINVAL error,
instead of a more sensible OpenFlow error.  This commit remedies the
situation.

Thanks to Justin for pointing out the issue.