V4L/DVB (6105): dvb_frontend: ts_bus_ctrl() handling fixed

ts_bus_ctrl() should only be called by
- the first open
- the last release call.

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