greybus: gb-audio: Reset sample count when CPort deactivated

The sample count placed in Greybus I2S audio messages should be
reset every time a new audio stream is set up.  However, the
current code does not do the reset so make it so it does.

Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>