i2c/ds1374: Check workqueue creation status

Check if workqueue creation failed.  Further usage of NULL pointed
workqueue is not good I guess ;)

Signed-off-by: Cyrill V. Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>