The SND_SOC_DAPM_PRE_PMU case would call startup()/hw_params() that
might access substream->runtime through other functions.

For example:

Unable to handle kernel NULL pointer dereference at virtual address
[....]
PC is at snd_pcm_hw_rule_add+0x24/0x1b0
LR is at snd_pcm_hw_constraint_list+0x20/0x28
[....]
Process arecord (pid: 424, stack limit = 0xffffffc1ecaf0020)
Call trace:
[<ffffffc00086be68>] snd_pcm_hw_rule_add+0x24/0x1b0
[<ffffffc00086c014>] snd_pcm_hw_constraint_list+0x20/0x28
[<ffffffc0008b47a4>] cs53l30_pcm_startup+0x24/0x30
[<ffffffc0008a6260>] snd_soc_dai_link_event+0x290/0x354
[<ffffffc0008a7528>] dapm_seq_check_event.isra.31+0x134/0x2c8
[<ffffffc0008a7768>] dapm_seq_run_coalesced+0x94/0x1c8
[<ffffffc0008a7940>] dapm_seq_run+0xa4/0x404
[<ffffffc0008a8bac>] dapm_power_widgets+0x524/0x984
[<ffffffc0008ab1c4>] snd_soc_dapm_stream_event+0x8c/0xa8
[<ffffffc0008ac7f4>] soc_pcm_prepare+0x10c/0x1ec
[<ffffffc000865b9c>] snd_pcm_do_prepare+0x1c/0x38
[<ffffffc000865600>] snd_pcm_action_single+0x40/0x88
[<ffffffc0008656b8>] snd_pcm_action_nonatomic+0x70/0x90
[<ffffffc000868d28>] snd_pcm_common_ioctl1+0xb6c/0xdd8
[<ffffffc000869508>] snd_pcm_capture_ioctl1+0x200/0x334
[<ffffffc00086a084>] snd_pcm_ioctl_compat+0x648/0x95c
[<ffffffc0001ff4b4>] compat_SyS_ioctl+0xac/0xfc4
[<ffffffc000084cf0>] el0_svc_naked+0x24/0x28
---[ end trace 0dc4f99c2759c35c ]---

So this patch adds a dummy runtime for the original dummy substream
to merely avoid the NULL pointer access.
Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>