ALSA: Fix the default suffix string with high card number

ALSA core tries to add a suffix as "_1" automatically when the given
id string conflicts.  The current code assumes implicitly that the max
card number is 16 so that the single hex "_X" suffix can be put.
However, with the dynamic device management, the card can be at most
32, so it can put even a non-hex character there.  Also, when the max
card number is increased in future, this would result in worse.

This patch rewrites the code to add the suffix string in a simpler
(thus cleaner) way.  It can support up to three digits, so it should
suffice for most requirements.

Signed-off-by: Takashi Iwai <tiwai@suse.de>