[WATCHDOG] fixed book E watchdog period register mask.

A previous fix changed the WDTP function to use the period directly,
rather than subtracting from 63.  However the mask generation was
not changed, so the mask was coming out as 0.  This patch fixes it.

Signed-off-by: Luuk Paulussen <luuk.paulussen@alliedtelesis.co.nz>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>