net: dsa: mv88e6xxx: Fix mv88e6xxx_g1_irq_free() interrupt count

mv88e6xxx_g1_irq_setup() sets up chip->g1_irq.nirqs interrupt mappings,
so free the same amount. This will be 8 or 9 in practice, less than 16.

Fixes: dc30c35b ("net: dsa: mv88e6xxx: Implement interrupt support.")
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>