V4L/DVB (11178): pvrusb2: Make audio sample rate update into a sub-device broadcast

The pvrusb2 driver had previously been using i2c module specific calls
to set the sample rate (a long long time ago this was needed).  These
days it is safe to use a broadcast so let's just broadcast this when
communicating audio sample rate to sub-devices.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>