timer/aspeed: fix timer enablement when a reload is not set

When a timer is enabled before a reload value is set, the controller
waits for a reload value to be set before starting decrementing. This
fix tries to cover that case by changing the timer expiry only when
a reload value is valid.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Message-id: 1496739312-32304-1-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
