Enable connecting devices to the root hub directly

[boochow]

This PR makes it possible to connect a USB FS/LS device to the root hub directly (without USB HS hub). It is useful if you want to connect a USB keyboard, for example, directly to the USB port of RPi model A/A+/Zero/Zero W because they have no on-board USB hub (LAN9512) and their USB port is the port of the root hub. Patches included in this PR stops USB SSPLIT/CSPLIT packets when a USB device connected to the root hub is a FS or LS device. I also added some lines to hub.c for detecting unplugging a device from the root hub.

[paulwratt]

@Chadderz121 can you please commit this PR, I'm using a fork of your repo and github does not allow 2 forks with the same master name.

@IchigoJam BASIC RPi may find this PR comment useful for [issue 1005] mentioned here: https://ichigojam.github.io/RPi/

as may many others (incl. @kelihlodversson pTOS project), specifically:

if you want to connect a USB keyboard, for example, directly to the USB port of RPi model A/A+/Zero/Zero W because they have no on-board USB hub (LAN9512) and their USB port is the port of the root hub.

NOTE: for "other RPi" this also implies that you need to check for "another hub layer" when using a wifi or bluetooth keyboard and mouse combo usb reciever (or one of PINE64 usb keyboard with 2 port hub).