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

net: dsa: microchip: ptp: 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, during the PTP IRQ setup,
we verify that its returned value isn't negative.

Fix the irq_find_mapping() check to enter the error path when 0 is
returned. Return -EINVAL in such case.

Cc: stable@vger.kernel.org
Fixes: cc13ab18b201 ("net: dsa: microchip: ptp: enable interrupt for timestamping")
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-v6-2-891f80ae7f8f@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>