irqchip/armada-370-xp: Use PCI_MSI_DOORBELL_START where appropriate

As suggested by Gregory Clement, this commit adjusts the
irq-armada-370-xp driver to use the PCI_MSI_DOORBELL_START define in
the armada_370_xp_handle_msi_irq() function, rather than hardcoding
its value.

Suggested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1455115621-22846-4-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>

drivers/irqchip/irq-armada-370-xp.c

@@ -401,10 +401,10 @@ static void armada_370_xp_handle_msi_irq(struct pt_regs *regs, bool is_chained)
 
 		if (is_chained) {
 			irq = irq_find_mapping(armada_370_xp_msi_inner_domain,
-					       msinr - 16);
+					       msinr - PCI_MSI_DOORBELL_START);
 			generic_handle_irq(irq);
 		} else {
-			irq = msinr - 16;
+			irq = msinr - PCI_MSI_DOORBELL_START;
 			handle_domain_irq(armada_370_xp_msi_inner_domain,
 					  irq, regs);
 		}