MAINTAINERS: update entry of firewire audio drivers

This commit adds myself as one of maintainers for firewire audio
drivers and IEC 61883-1/6 packet streaming engine. I call them ALSA
firewire stack as a whole.

6 years ago I joined in development for this category of drivers with
heavy reverse-engineering tasks and over 100 models are now available
from ALSA applications. IEEE 1394 bus itself and units on the bus are
enough legacy but the development still continues.

I have a plan to add drastic enhancement in kernel v5.5 and v5.6 period.
This commit adds myself into MAINTAINERS so that developers and users
can easily find active developer to post their issues, especially for
regression.

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