V4L/DVB (4013): Change the sweeprate for TT C1500 using QAM64

This patch changes the sweeprate for TT C1500 using QAM64.
It has been proven to work using QAM64 at a SRate of 6875 for the two

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>