OMAP: DSS: Taal: fix error returns in taal_probe()

The workqueue creation error branch attempted to destroy a NULL wq,
and, in turn, a failed registration does not destroy the newly created
workqueue.

The problem was reported by a static analysis tool.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>