V4L/DVB (10620): gspca - main: More checks of the device disconnection.

- prevent application oops when the device is disconnected
- wake up the application at disconnection time
- check the disconnection in ioctl dqbuf and poll

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>