[usb] Rename "USB device" to "USB peripheral" when referring to USB peripheral role

Before this CL, "USB device" could mean two things:

1) a USB device attached to Zircon in USB host role, or
2) Zircon acting as a USB device attached to another host.

This change renames the 2) use case to "USB peripheral" to make the two roles
more clearly defined.

TEST: manual testing on Hikey, switching roles between host and peripheral
with the usbctl tool. (Hikey is currently the only platform that supports this).

Change-Id: Ic0143f51d7616fae1f04d298081dcf22a2c232fa