ALSA: pcxhr: fix string overflow warnings

With gcc-7, we get a warning about a possible string overflow:

sound/pci/pcxhr/pcxhr.c: In function 'pcxhr_probe':
sound/pci/pcxhr/pcxhr.c:1647:28: error: ' [PCM #' directive writing 7 bytes into a region of size between 1 and 32 [-Werror=format-overflow=]

The shortname can simply be removed, and the longname can
be changed into a shorter "name" string that is used in three
places. Making it a little shorter (40 bytes) avoids the risk of
overflowing completely, but I also use snprintf() here for
extra clarity.

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