powerpc/mpic: Fix incorrect allocation of interrupt rev-map

Before when we were setting up the irq host map for mpic we passed in
just isu_size for the size of the linear map.  However, for a number of
mpic implementations we have no isu (thus pass in 0) and will end up
with a no linear map (size = 0).  This causes us to always call
irq_find_mapping() from mpic_get_irq().

By moving the allocation of the host map to after we've determined the
number of sources we can actually benefit from having a linear map for
the non-isu users that covers all the interrupt sources.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>