MAINTAINERS: update entry for firewire audio drivers with UAPI header

The most of drivers in ALSA firewire stack supports common ioctl commands
to enable/disable packet streaming as well as some ioctl commands for
model-specific features. An UAPI header is exported to userspace.

This commit adds supplement for entry of ALSA firewire stack with a path
of the UAPI header.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>