V4L/DVB (9690): gspca: Lock the subdrivers via module_get/put.

The previous subdriver protection against rmmod was done via the
file operations table in the device descriptor. On device disconnection
while streaming, the device structure was freed at close time, and the
module_put still used the module name in the freed area.
Now, explicit module get/put are done on open and close.

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