V4L/DVB (6475): Fix some troubles at list handling

- priv->count were wrong. Should be incremented since the first usage;
- forgot to use list_del() to remove the driver;
- Release memory if an error occurs during _attach

Thanks to Aidan Thornton <makosoft@googlemail.com> for pointing this.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>