PCI: designware: Fix missing MSI IRQs

The interrupts were cleared after the IRQ handler was called.  This means
that new interrupts that occur after the handler handled the previous IRQ
but before the interrupt is cleared will be missed.

Tested-by: Marek Vasut <marex@denx.de>
Tested-by: Matthias Mann <m.mann@arkona-technologies.de>
Signed-off-by: Harro Haan <hrhaan@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Mohit Kumar <mohit.kumar@st.com>
Cc: Richard Zhu <hong-xing.zhu@freescale.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Pratyush Anand <pratyush.anand@st.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Juergen Beisert <jbe@pengutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Siva Reddy Kallam <siva.kallam@samsung.com>
Cc: Srikanth T Shivanand <ts.srikanth@samsung.com>
Cc: Sean Cross <xobs@kosagi.com>