net: dsa: mv88e6xxx: Add helper to get a chip's max_vid

Most of the other chip info constants have helpers to get at them; add
one for max_vid to keep things consistent.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201110185720.18228-1-tobias@waldekranz.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>