clocksource: timer-keystone: Delete unnecessary variable

Commit 438e0bff5257 added the timer-keystone device driver but make use
of an unnecessary variable in the init function. This patch deletes this
variable.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>