fix an assert in veb_p_ioctl() that failed when called by a span port.

fix an assert in veb_p_ioctl() that failed when called by a span port.

veb_p_ioctl() is used by both veb bridge and veb span ports, but
it had an assert to check that it was being called by a veb bridge
port. this extends the check so using it on a span port doesnt cause
a panic.

found by dave voutila