net: dsa: microchip: ksz8795: use num_vlans where possible

The value of the define VLAN_TABLE_ENTRIES can be derived from
num_vlans. This patch is using the variable num_vlans instead and
removes the extra define.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>