[POWERPC] PS3: Use clear_bit

Replace the inline asm with bitops in the PS3 interrupt
chip mask routines.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>