[POWERPC] mpc52xx_pic: fix main interrupt masking

The bit setting was off by one.
Tested with RTC and GPIO_WKUP interrupts.

Signed-off-by: Domen Puncer <domen.puncer@telargo.com>
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>