mlxsw: spectrum: Fix base port get for split count 4 and 8

The current code considers only split by 2 or 4. Make the base port
getting generic and allow split by 8 to be handled correctly. Generalize
the used port checks as well.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Shalom Toledo <shalomt@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>