games/linux-enemyterritory: Fix sound

The game starts writing audio s_mixPreStep seconds into the sound
buffer, but s_mixPreStep defaults to 0.05 (50ms), and sound(4) hands out
a ~46ms buffer by default (hw.snd.latency=2), so 50ms is already past
the end of the buffer and nothing is ever written. Start writing at 20ms
instead, which leaves space for sound to be written.

Approved by: arrowd, arved
PR: 251660
Sponsored by: The FreeBSD Foundation