ALSA: hdspm - Constrain periods to 2 on older cards

RME RayDAT and AIO use a fixed buffer size of 16384 samples. With period
sizes of 32-4096, this translates to 4-512 periods.

The older RME cards have a variable buffer size but require exactly two
periods.

This patch enforces nperiods=2 on those cards.

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Cc: <stable@vger.kernel.org> # 2.6.39+
Signed-off-by: Takashi Iwai <tiwai@suse.de>