alsactl: fix error handling for sched_setscheduler() call

alsactl: fix error handling for sched_setscheduler() call

As man page says: "If successful, the sched_setparam() function shall return zero."

Without update I got this output in the syslog (journalctl):
abr 16 09:25:30 mypc alsactl[1652]: alsactl 1.2.2 daemon started
abr 16 09:25:30 mypc alsactl[1652]: /usr/bin/alsactl: do_nice:165sched_setparam failed: No such file or directory

If sched_setscheduler() returns 0, so it means that the call was successful.

Signed-off-by: Oscar Megía <megia.oscar@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>