net: dsa: microchip: common: Fix checks on irq_find_mapping()

net: dsa: microchip: common: Fix checks on irq_find_mapping()

irq_find_mapping() returns a positive IRQ number or 0 if no IRQ is found
but it never returns a negative value. However, on each
irq_find_mapping() call, we verify that the returned value isn't
negative.

Fix the irq_find_mapping() checks to enter error paths when 0 is
returned. Return -EINVAL in such cases.

CC: stable@vger.kernel.org
Fixes: c9cd961c0d43 ("net: dsa: microchip: lan937x: add interrupt support for port phy link")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Bastien Curutchet (Schneider Electric) <bastien.curutchet@bootlin.com>
Link: https://patch.msgid.link/20251120-ksz-fix-v6-1-891f80ae7f8f@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>