Only build USB-related modules if MK_USB != no

This change moves the thunderbolt module and other USB modules under a
MK_USB != no conditional to ensure that users not desiring USB support
can easily build systems without USB-specific drivers using this knob.

MFC after: 1 week
Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D55576

(cherry picked from commit c47cefba831240a1b3de375f18134b93cf998f5c)