[media] cx88: kernel bz#9476: Fix tone setting for Nova-S+ model 92001

[media] cx88: kernel bz#9476: Fix tone setting for Nova-S+ model 92001

Hauppauge Nova-S-Plus DVB-S model 92001 does not lock on horizontal
polarisation. According with the info provided at the BZ, model
92002 does.

The difference is that, on model 92001, the tone select is done via
isl6421, while, on other devices, this is done via cx24123 code.
This patch adds a way to override the demod's set_tone at isl6421
driver. In order to avoid regressions, the override is enabled
only for cx88 Nova S plus model 92001. For all other models and
devices, the set_tone is provided by the demod driver.

Patch originally proposed at bz@9476[1] by Michel Meyers and
John Donoghue but applying the original patch would break support
for all other devices based on isl6421.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=9476

Tested-by: Adam Sampson <ats@offog.org>
Tested-by: Hans-Peter Jansen <hpj@urpla.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>