ALSA: hdsp: allow firmware loading from inside the kernel

Allow the use of the FIRMWARE_IN_KERNEL option with hdsp cards and
in-kernel driver.
Also corrected a typo in the comment.

Signed-off-by: Raphael Doursenaud <rdoursenaud@free.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>