media: ddbridge: stv0910 single demod mode module option

Adds a stv0910_single modparm which, when set, configures the stv0910 to
run in single demodulator mode, currently intended for high bit rate
testing.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>