mfd: twl-core: Fix clock initialization

When looking up the clock we must use the client->dev as device since that
is the one which is probed via DT.

Cc: stable@vger.kernel.org # 4.16+
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>