[POWERPC] qe_ic: Do a sync when masking interrupts

This patch causes a sync do be done after masking a QE interrupt, to
ensure that the masking has completed before interrupts are enabled.
This allows the masking of the cascade IRQ to be removed without causing
spurious interrupts.

The mask_and_ack function is also removed and set to the mask function,
as the two are identical.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>