ALSA: als4000: enable burst mode

Enable burst mode to prevent dropouts during high PCI bus usage.
The card is useless in X without this because of dropouts when anything moves
on the screen (at least with PCI VGA card). Enabling this is also recommended
by the datasheet (page 48).

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>