usb: Add missing mtx lock and unlock in pushing dma queue

Accessing usb_xfer_queue requires bus lock, we added this missing lock
in here to prevent racing issue.

Reviewed by: adrian
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57293

(cherry picked from commit df5e9e3da5b9b3fe63ed4aaaa19b824fd18ae0f2)