powerpc: Fix Book-E watchdog timer interval setting

This patch fixes the setting of the Book-E watchdog timer interval setup
on initialization and by ioctl().

On initialization the period bits have to be masked before setting
a new period.

In WDIOC_SETTIMEOUT ioctl we have to use the correct mask.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>