ASoC: dmic: Fix NULL pointer dereference

Regulator support was introduced in commit d3321a20 ("ASoC: dmic: add
regulator support"). During probe `dmic->vref` is initialized with
devm_regulator_get_optional() but in the error flow doesn't get cleared in
the case that PTR_ERR(dmic->vref) is -ENODEV. This leads to the following
NULL pointer deref.

```
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 7 UID: 1000 PID: 1587 Comm: wireplumber Not tainted 6.14.0-rc7-next-20250318 #1 PREEMPT(voluntary)
RIP: 0010:regulator_enable+0x17/0x70
RSP: 0018:ffffcc10c1fe7a38 EFLAGS: 00010282
RAX: ffff8bccc1c25010 RBX: ffffffffffffffed RCX: 0000000000000000
RDX: 0000000000000002 RSI: ffffcc10c1fe7a38 RDI: ffffffffffffffed
RBP: ffffcc10c1fe7a68 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8bcccd51f800
R13: ffffffffc1086e88 R14: 0000000000000001 R15: 0000000000000001
FS:  00007f927bc35800(0000) GS:ffff8bd44f09f000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000065 CR3: 00000001332c6000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
 <TASK>
 ? show_regs+0x6c/0x80
 ? __die+0x24/0x80
 ? page_fault_oops+0x154/0x570
 ? hrtimer_start_range_ns+0x142/0x4e0
 ? timerqueue_del+0x31/0x50
 ? do_user_addr_fault+0x4ac/0x880
 ? exc_page_fault+0x82/0x1d0
 ? asm_exc_page_fault+0x27/0x30
 ? regulator_enable+0x17/0x70
 ? __schedule+0x491/0x16b0
 dmic_aif_event+0x82/0xa0 [snd_soc_dmic]
```

Adjust the error flow to explicitly set it back to NULL to avoid calling
regulator_enable() with garbage data.

Reported-by: Akshata V Unkal <Akshata.VUnkal@amd.com>
Fixes: d3321a20 ("ASoC: dmic: add regulator support")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/20250319145636.2401680-1-superm1@kernel.org


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