Make the handler routine for the hw.usb.template sysctl trigger the USB
host to reprobe the bus by switching the USB pull up resistors off and
back on. In other words - when FreeBSD is configured as a USB device,
changing the sysctl will be immediately noticed by the machine it's
connected to.

Reviewed by: hselasky@
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation