sound: Fix oss_sysinfo->numcards

According to the OSS manual, oss_sysinfo->numcards holds the number of
detected audio devices in the system, while the current ncards variable,
whose value is assigned to oss_sysinfo->numcards, holds the number of
currently registered devices only.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dev_submerge.ch, emaste
Differential Revision: https://reviews.freebsd.org/D45136