net: dsa: mv88e6xxx: Create helper for FIDs in use

Refactor the code in mv88e6xxx_atu_new() which builds a bitmaps of
FIDs in use into a helper function. This will be reused by the devlink
code when dumping the ATU.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>