ALSA: usb-audio: mixer: Remove temporary string use in parse_clock_source_unit

The kctl->id.name can be directly passed to snd_usb_copy_string_desc() and
if the string has been fetched the suffix can be appended with the
append_ctl_name() call.
The temporary name string becomes redundant and can be removed.

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

sound/usb/mixer.c: In function ‘parse_audio_unit’:
sound/usb/mixer.c:1972:29: error: ‘ Validity’ directive output may be truncated writing 9 bytes into a region of size between 1 and 44 [-Werror=format-truncation=]
 1972 |                          "%s Validity", name);
      |                             ^~~~~~~~~
In function ‘parse_clock_source_unit’,
    inlined from ‘parse_audio_unit’ at sound/usb/mixer.c:2892:10:
sound/usb/mixer.c:1971:17: note: ‘snprintf’ output between 10 and 53 bytes into a destination of size 44
 1971 |                 snprintf(kctl->id.name, sizeof(kctl->id.name),
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1972 |                          "%s Validity", name);
      |                          ~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

The warnings got brought to light by a recent patch upstream:
commit 6d4ab2e9

 ("extrawarn: enable format and stringop overflow warnings in W=1")

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20230913093933.24564-1-peter.ujfalusi@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>