powerpc: Fix i8259 cascade on pSeries with XICS interrupt controller

It turns out that commit f9bd170a
broke the cascade from XICS to i8259 on pSeries machines; specifically
we ended up not ever doing the EOI on the XICS for the cascade.  The
result was that interrupts from the serial ports (and presumably any
other devices using ISA interrupts) didn't get through.  This fixes
it and also simplifies the code, by doing the EOI on the XICS in the
xics_get_irq routine after reading and acking the interrupt on the
i8259.

Signed-off-by: Paul Mackerras <paulus@samba.org>