Add a tsspeed config option to struct stv0910_cfg which can be used by
users of the driver to set the (parallel) TS speed (higher speeds enable
support for higher bitrate transponders). If tsspeed isn't set in the
config, it'll default to a sane value.

This commit also updates the two consumers of the stv0910 driver (ngene
and ddbridge) to have a default tsspeed in their stv0910_cfg templates.
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <rascobie@slingshot.co.nz>
Tested-by: Helmut Auer <post@helmutauer.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>