greybus: audio: Reorder gb_deactivate sequence to avoid protocol error

gb_activate_tx/rx is triggered from _prepare() & gb_deactivate
from shutdown(). This may cause protocol error in case shutdown
executes without _prepare due to some hw_params failure.

Also, reorganise _prepare & _shutdown calls to make it more
readable & cleaner.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>