pcm: multi: Drop the fixed slave_map[] in snd_pcm_multi_open()

slave_map[] in snd_pcm_multi_open() is a fixed size array and
obviously we have no overflow check, and eventually the program gets
an error when more than 64 channels are used.

Although we can modify the code to allocate the array dynamically, it
turned out that we can drop the whole slave_map[] thingy in this
function when looking at the code closely.  In the past, it was used
to identify the one-to-many mapping.  But the check was dropped, and
now it's nothing more than a sanity check.

Reported-by: Jörg Müller <joerg.mueller7744@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>