ASoC: mediatek: common: handle NULL case in suspend/resume function

ASoC: mediatek: common: handle NULL case in suspend/resume function

When memory allocation for afe->reg_back_up fails, reg_back_up can't
be used.
Keep the suspend/resume flow but skip register backup when
afe->reg_back_up is NULL, in case illegal memory access happens.

Fixes: e6f1913974da ("ASoC: mediatek: implement mediatek common structure")
Signed-off-by: Trevor Wu <trevor.wu@mediatek.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210910092613.30188-1-trevor.wu@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>