ASoC: fsl: imx-pcm-rpmsg: remove useless initialization

cppcheck warning:

assigned a value that is never used. [unreadVariable]
 int written_num = 0;
                 ^

sound/soc/fsl/imx-pcm-rpmsg.c:547:18: style: Variable 'written_num' is
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210426214701.235106-5-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>