ALSA: control: remove useless assignment in .info callback of PCM chmap element

ALSA: control: remove useless assignment in .info callback of PCM chmap element

Control elements for PCM chmap return information to userspace abount
the maximum number of available PCM channels as the number of values
in the element.

In current implementation the number is once initialized to zero, then
assigned to. This is useless and this commit fixes it.

Fixes: 2d3391ec0ecc ("ALSA: PCM: channel mapping API implementation")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191214131351.28950-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>