MAINTAINERS: update git tree for sound entries

Several sound related entries in MAINTAINERS refer to the old git tree
at "git://git.alsa-project.org/alsa-kernel.git".  This is no longer used
for development, and Takashi Iwai's kernel.org tree is used instead.

Signed-off-by: Ross Zwisler <zwisler@google.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>