ALSA: snd-aloop - fix issue in the timer start function

In some circumstances (the rate shift value was changed), the irq_pos
value may be higher than the fraction value in the timer start function.
Check for it.

Also, to avoid value overflow, decrease maximum period size.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

diff --cc sound/drivers/aloop.c
Simple merge