vfio: platform: Switch to platform_get_irq_optional()

Since commit 7723f4c5 ("driver core: platform: Add an error
message to platform_get_irq*()"), platform_get_irq() calls dev_err()
on an error. As we enumerate all interrupts until platform_get_irq()
fails, we now systematically get a message such as:
"vfio-platform fff51000.ethernet: IRQ index 3 not found" which is
a false positive.

Let's use platform_get_irq_optional() instead.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Cc: stable@vger.kernel.org # v5.3+
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>