Add a tunable sysctl, hw.usb.uaudio.handle_hid, to allow disabling the
the HID volume keys support in the USB audio driver.

While at it re-organize the USB audio sysctls a bit.

Differential Revision: https://reviews.freebsd.org/D27180
MFC after: 1 week
Sponsored by: Mellanox Technologies // NVIDIA Networking