V4L/DVB (6106): dvb_frontend: Default for dvb_shutdown_timeout set to 0

Default for dvb_shutdown_timeout set to 0.
dvb_shutdown_timeout > 0 is used for debugging only.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>