There is no need to use temporary string for the debugfs directory name as
we can use the device name of the card.

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

sound/core/init.c: In function ‘snd_card_init’:
sound/core/init.c:367:28: error: ‘%d’ directive writing between 1 and 10 bytes into a region of size 4 [-Werror=format-overflow=]
  367 |         sprintf(name, "card%d", idx);
      |                            ^~
sound/core/init.c:367:23: note: directive argument in the range [0, 2147483646]
  367 |         sprintf(name, "card%d", idx);
      |                       ^~~~~~~~
sound/core/init.c:367:9: note: ‘sprintf’ output between 6 and 15 bytes into a destination of size 8
  367 |         sprintf(name, "card%d", idx);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

The idx is guarantied to be less than SNDRV_CARDS (max 256 or 8) by the
code in snd_card_init(), however the compiler does not see that.

The warnings got brought to light by a recent patch upstream:
commit 6d4ab2e9 ("extrawarn: enable format and stringop overflow warnings in W=1")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20230912110113.3166-1-peter.ujfalusi@linux.intel.comSigned-off-by: Takashi Iwai <tiwai@suse.de>