irqchip: armada-370-xp: Fix releasing of MSIs

Store the value of d->hwirq in a local variable as the real value is wiped out
by calling irq_dispose_mapping. Without this patch, the armada_370_xp_free_msi
function would always free MSI#0, no matter what was passed to it.

Fixes: 31f614ed

 ('irqchip: armada-370-xp: implement MSI support')
Cc: <stable@vger.kernel.org> # v3.13+
Signed-off-by: Neil Greatorex <neil@fatboyfat.co.uk>
Link: https://lkml.kernel.org/r/1397823593-1932-4-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1397823593-1932-4-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>