net: thunderbolt: Add comment clarifying prtcstns flags

ThunderboltIP protocol currently has two flags from which we only
support and set match frags ID. The first flag is reserved for full E2E
flow control. Add a comment that clarifies them.

Suggested-by: Yehezkel Bernat <yehezkelshb@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>