Add dsp setup related functions for the Sound Blaster Z, along with
other helper functions.

Also, add sbz_dsp_startup_check, which fixes a bug where the card
sometimes starts up and has no sound.
Signed-off-by: Connor McAdams <conmanx360@gmail.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>