V4L/DVB (3973): Cx88-blackbird: pause the encoder during frequency change

Added code to cx88-blackbird.c to treat the VIDIOC_S_FREQUENCY ioctl
to stop mpeg stream before changing the frequency and restart it after.
It seems that the mpeg stream needs to be paused while changing the
channel frequency, otherwise the mpeg encoder enters into a bad state,
producing artifacts.

Signed-off-by: Valentin Zagura <puthre@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>