V4L/DVB (4504): Enable audio DMA restart on channel change even when cx88-alsa is compiled

cx88-alsa can be compiled in (in distro built kernels, for example), but
not used. In those cases, the audio DMA restart is needed for proper sound
on NICAM channels.
This patch enables the DMA restart even with ALSA, but the functions now
check if cx88-alsa is really active.

Signed-off-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>