ARM: twd: ensure timer reload is reprogrammed on entry to periodic mode

Ensure that the twd timer reload value is reprogrammed each time we
enter periodic mode.  This ensures that the reload value is always
reset correctly.

Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Colin Cross <ccross@android.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>