[ALSA] wavefront - Use standard firmware loader

Use the standard firmware loader for loading ICS2115 OS firmware file.
This is the last old bad guy that is still using sys_open() and sys_read()
calls, and now all should be gone.
The patch also adds the missing description of module options related
with wavefront_synth.c.
Due to this rewrite, user will have to copy or make symlink the firmware
file appropriately to the standard firmware path such as /lib/firmware.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>