sound: Simplify unit fetching in dsp_oss_audioinfo()

"i" keeps the value of the current unit, so we do not have to call
PCMUNIT() and device_get_unit() to fetch it.

In the mixer case, I think it is more correct to do it like this, since
mixer and DSP device units have a 1-1 relationship (i.e the mixer unit
is always the same as the corresponding DSP device one) and that way we
can make it more clear.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44855