ALSA: hda: intel-sdw-acpi: Use u8 type for link index

Use consistently u8 for sdw link index. The id is limited to 4, u8 is
adequate in size to store it.

This change will also fixes the following compiler warning/error (W=1):

sound/hda/intel-sdw-acpi.c: In function ‘sdw_intel_acpi_scan’:
sound/hda/intel-sdw-acpi.c:34:35: error: ‘-subproperties’ directive output may be truncated writing 14 bytes into a region of size between 7 and 17 [-Werror=format-truncation=]
   34 |                  "mipi-sdw-link-%d-subproperties", i);
      |                                   ^~~~~~~~~~~~~~
In function ‘is_link_enabled’,
    inlined from ‘sdw_intel_scan_controller’ at sound/hda/intel-sdw-acpi.c:106:8,
    inlined from ‘sdw_intel_acpi_scan’ at sound/hda/intel-sdw-acpi.c:180:9:
sound/hda/intel-sdw-acpi.c:33:9: note: ‘snprintf’ output between 30 and 40 bytes into a destination of size 32
   33 |         snprintf(name, sizeof(name),
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
   34 |           ...