V4L/DVB: cx18-alsa: fix memory leak in error condition

If the stream is already in use, make sure we free up the memory allocated
earlier.

Thanks to Andy Wall for reviewing and pointing this out.

This work was sponsored by ONELAN Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>