ALSA: mixart: fix string overflow warning

ALSA: mixart: fix string overflow warning

Using a temporary string produces warnings about a possible overflow
in sprintf:

sound/pci/mixart/mixart.c: In function 'snd_mixart_probe':
sound/pci/mixart/mixart.c:1353:28: error: ' [PCM #' directive writing 7 bytes into a region of size between 1 and 32 [-Werror=format-overflow=]
   sprintf(card->shortname, "%s [PCM #%d]", mgr->shortname, i);
                            ^~~~~~~~~~~~~~
sound/pci/mixart/mixart.c:1353:28: note: using the range [-2147483648, 2147483647] for directive argument
sound/pci/mixart/mixart.c:1353:3: note: 'sprintf' output between 10 and 51 bytes into a destination of size 32
   sprintf(card->shortname, "%s [PCM #%d]", mgr->shortname, i);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/pci/mixart/mixart.c:1354:27: error: ' [PCM #' directive writing 7 bytes into a region of size between 1 and 80 [-Werror=format-overflow=]
   sprintf(card->longname, "%s [PCM #%d]", mgr->longname, i);
                           ^~~~~~~~~~~~~~
sound/pci/mixart/mixart.c:1354:27: note: using the range [-2147483648, 2147483647] for directive argument
sound/pci/mixart/mixart.c:1354:3: note: 'sprintf' output between 10 and 99 bytes into a destination of size 80

Skipping the intermediate, we can get gcc to see that it is in fact
safe here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>