1.2.16 upgrade: could not open configuration file

alsa-project/alsa-lib issue #510 was opened from tekstryder:

After upgrading to the latest tagged release [v1.2.16](https://github.com/alsa-project/alsa-ucm-conf/releases/tag/v1.2.16), I'm seeing the following errors logged:

```
Jun 04 08:01:53 alsactl[721]: alsa-lib utils.c:372:(uc_mgr_config_load_into) could not open configuration file /usr/share/alsa/ucm2/USB-Audio/conf.d/0db0-1feb.conf
Jun 04 08:01:53 alsactl[721]: alsa-lib parser.c:79:(uc_mgr_config_load_file) failed to open file /usr/share/alsa/ucm2/USB-Audio/conf.d/0db0-1feb.conf: -2
Jun 04 08:06:02 pipewire[981]: ALSA lib utils.c:372:(uc_mgr_config_load_into) [error.ucm] could not open configuration file /usr/share/alsa/ucm2/USB-Audio/conf.d/0db0-1feb.conf
Jun 04 08:06:02 pipewire[981]: ALSA lib parser.c:79:(uc_mgr_config_load_file) [error.ucm] failed to open file /usr/share/alsa/ucm2/USB-Audio/conf.d/0db0-1feb.conf: -2
```

The device was integrated into `../ucm2/USB-Audio/USB-Audio.conf` long ago.

No such file should be searched for by the parser logic if the `/conf.d` path does not exist.

```
/usr/share/alsa ❯ grep -r 1feb      
ucm2/USB-Audio/USB-Audio.conf: # 0db0:1feb MSI Edge Wifi Z690
ucm2/USB-Audio/Realtek/ALC4080-HiFi.conf: Regex "USB(0b05:(1996|1a5[23c])|0db0:1feb)"
```

Never had a single issue with this device functionality since we first got it working in https://github.com/alsa-project/alsa-ucm-conf/issues/210 years ago.

https://github.com/alsa-project/alsa-ucm-conf/commit/1bb0739acfc9bbfe61ffaf4707ea9a9d98cdb82b seems responsible:

```
/usr/share/alsa ❯ grep conf.d /usr/share/alsa/ucm2/USB-Audio/USB-Audio.conf
# Optional configuration can be stored to USB-Audio/conf.d/{VENDOR}-{DEVICE}.conf files
File "/USB-Audio/conf.d/${var:USBID1}-${var:USBID2}.conf"
```

https://github.com/alsa-project/alsa-ucm-conf/blob/f846a8bba8fc2d21d23caf7eafab5f560f9196fa/ucm2/USB-Audio/USB-Audio.conf#L278-L281

Should check whether folder path `"/USB-Audio/conf.d/"` exists first.

This causes spurious log noise. The device audio is working as intended.

Issue URL     : https://github.com/alsa-project/alsa-lib/issues/510
Repository URL: https://github.com/alsa-project/alsa-lib