microblaze: avoid interrupt race conditions

The interrupt acknowledge action have to run after the
registered interrupt handler. So we have a chance to
bear out the corresponding interrupt request in the
corresponding controller hardware.

With this reordering, we optain a proper interrupt
handling for level triggered interrupt sources -- for
example the new axi_timer v1.02.a introduced in ISE 13.2.

Signed-off-by: Stephan Linz <linz@li-pro.net>
Acked-by: Michal Simek <monstr@monstr.eu>