[ALSA] timer: fix timer rescheduling

When checking whether a hardware timer needs to be rescheduled, we have
to compare against the previously scheduled interval and not against the
actual interval between the last two interrupts.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>