ALSA: hdsp - Add a comment about external firmwares for hdsp

When the hdsp driver is built in kernel, the corresponding firmware
files have to be built into the kernel as well (otherwise the boot
will hang up for fairly long time), but there is no way to control
it in Kconfig yet, unfortunately.  Instead, show a comment for user
just to make sure.

Signed-off-by: Takashi Iwai <tiwai@suse.de>