pci: pcie-designware: Remove irq_desc abuse

There is no reason to care about irq_desc in that context, escpecially
as irq_data for that interrupt is retrieved as well.

Use the proper accessor for the msi descriptor

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Cc: Mohit Kumar <mohit.kumar@st.com>
Cc: pci <linux-pci@vger.kernel.org>
Link: http://lkml.kernel.org/r/20140223212736.987803648@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>