sound: Retire snd_midi->qlock

snd_midi->qlock is used to protect snd_midi->{in,out}q. However, apart
from the numerous LORs present already in the code, there is no reason
not to use snd_midi->lock, as we do for the rest of the structure

Started by: https://github.com/freebsd/freebsd-src/pull/1902
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_nicolas-provost.fr
Differential Revision: https://reviews.freebsd.org/D54129