V4L/DVB (5716): Tda10086,tda826x: fix tuning, STR/SNR values

Several people reported unreliable reception with the current driver.
Furthermore, STR and SNR values seem to be inverted.
This fix is based on a patch posted by Hartmut Hackman.
Thanks to Helmut Auer for testing and helping to optimize the patch.
tda826x:
- set baseband cut-off to 19 MHz
tda10086:
- change the parameters of the carrier recovery loop
- toggle register 0x02 between 0x35 (tuning) and 0x00 (locked)
- invert STR and SNR values

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