V4L/DVB (12818): tm6000: Use dvb_frontend_detach instead of single symbol_put commands

Thanks to Mike Krufky for pointed that one out. Also include some minor
cleanups.

Signed-off-by: Michel Ludwig <michel.ludwig@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>