greybus: gb-audio: Set samples per message during init

A recent commit moved the I2S samples per message operation
to the PCM's 'hw_params' callback.  However, the 'hw_params'
callback is called numerous times while the samples per
message need only be done once (or seldom).  Eliminate the
unnecessary samples per message operations by doing it only
once at Greybus protocol init time.

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