sound: Get rid of snddev_info->devcount

snddev_info->devcount keeps track of the total number of channels for a
given device. However, it is redundant to have it, since it is only used
in sound_oss_sysinfo() to populate the "numaudios" field, and we also
keep track of the channel counts in the playcount, pvchancount, reccount
and rvchancount fields anyway. We can simply sum those fields together
instead of updating a separate variable upon every channel
addition/deletion.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44852