V4L/DVB (6666): saa7134-alsa: fix period handling

The period handling in saa7134-alsa is broken in two ways. First, the
minimum number of periods of two does not work, because the dma is setup
two periods ahead in the irq handler. Fix the minimum to four periods.
Second, the code assumes that the number of periods is divisible by two,
which isn't always the case on ALSA. Fix by adding a constraint.

Signed-off-by: Heikki Lindholm <holindho@cs.helsinki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>