ALSA: hdsp: improve firmware caching

As the recent firmware code tries to reread and cache the firmware by
itself, it's even better to keep the struct firmware data instead of
keeping a local copy.  Also, it makes little sense to disable the fw
loader for this driver, so added the explicit dependency, too.

Last, but not least, allocate the firmware data loaded via ioctl in
vmalloc'ed buffer instead, as the firmware size isn't that small.

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