sound: Remove ncards variable from sound_oss_card_info()

The loop counter is also the card's index, so ncards is redundant.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45144

(cherry picked from commit 305db91d4b92a5d53826dcb6df39fc8a1f3b427e)