[POWERPC] PS3: Fix unlikely typo in ps3_get_irq

Fix a typo bug 'unlikely(x) == y' and add an unlikely() call to
an unlikely code path in the PS3 interrupt routine ps3_get_irq().

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>