[ALSA] system timer: fix lost ticks correction adjustment

Fix the adjustment of the lost ticks correction variable in the case
when the correction has been fully taken into account in the next timer
expiration value.  Subtracting the scheduled ticks value would result in
an underflow.

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