usb: typec: unlock dp->lock on error exit path, and also zero ret if successful

One of the error handling paths forgets to unlock dp->lock on the error
exit path leading to a potential lock-up.  Also the return path for a
successful call to the function configuration_store can return an
uninitialized error return code if dp->alt->active is false, so ensure
ret is zeroed on the successful exit path to avoid garbage being returned.

Detected by CoverityScan, CID#1471597 ("Unitialized scalar variable")

Fixes: 0e3bb7d6 ("usb: typec: Add driver for DisplayPort alternate mode")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>