pm2301_charger: Return error if create_singlethread_workqueue fails

Return error instead of 0 if create_singlethread_workqueue call fails.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>