net: dsa: mv88e6xxx: Fix sparse warnings from GENMASK

Oddly, GENMASK() requires signed bit numbers, so that it can compare
them for < 0. If passed an unsigned type, we get warnings about the
test never being true.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.h

@@ -167,7 +167,7 @@ struct mv88e6xxx_irq {
 	u16 masked;
 	struct irq_chip chip;
 	struct irq_domain *domain;
-	unsigned int nirqs;
+	int nirqs;
 };
 
 /* state flags for mv88e6xxx_port_hwtstamp::state */
@@ -654,7 +654,7 @@ static inline unsigned int mv88e6xxx_num_ports(struct mv88e6xxx_chip *chip)
 
 static inline u16 mv88e6xxx_port_mask(struct mv88e6xxx_chip *chip)
 {
-	return GENMASK(mv88e6xxx_num_ports(chip) - 1, 0);
+	return GENMASK((s32)mv88e6xxx_num_ports(chip) - 1, 0);
 }
 
 static inline unsigned int mv88e6xxx_num_gpio(struct mv88e6xxx_chip *chip)