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: NTakashi Iwai <tiwai@suse.de>

sound/pci/Kconfig

@@ -532,6 +532,9 @@ config SND_HDSP
 	  To compile this driver as a module, choose M here: the module
 	  will be called snd-hdsp.
 
+comment "Don't forget to add built-in firmwares for HDSP driver"
+	depends on SND_HDSP=y
+
 config SND_HDSPM
 	tristate "RME Hammerfall DSP MADI"
 	select SND_HWDEP