powerpc/mpic: Remove WHOAMI readback after EOI

After previous discussions regarding the subject [1][2], there's no clear
explanation or reason why the call was needed in the first place. The sensible
argument is some sort of synchronization between the CPU and the MPIC, which
hasn't been pointed out precisely and is no longer required (at least on BookE
platforms).

The benefit of this change is saving a MMIO trap per interrupt when running in a
KVM guest.

[1] https://patchwork.ozlabs.org/patch/429098/
[2] https://patchwork.ozlabs.org/patch/433557/



Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>