sndctl(8): Fix dp->realtime

Currently it's automatically set to 0 in read_dev() as a result of
allocating dp with calloc().

Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50400

(cherry picked from commit 30fd79b0c0a328536b166e7fa9170b059e711303)