usb: Makefile cleanup

Group files, sprinkle in some comments.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/Makefile.objs

+# usb subsystem core
+common-obj-y += core.o combined-packet.o bus.o desc.o
+common-obj-y += libhw.o
+
+# usb host adapters
 common-obj-$(CONFIG_USB_UHCI) += hcd-uhci.o
 common-obj-$(CONFIG_USB_OHCI) += hcd-ohci.o
 common-obj-$(CONFIG_USB_EHCI) += hcd-ehci.o hcd-ehci-pci.o hcd-ehci-sysbus.o
 common-obj-$(CONFIG_USB_XHCI) += hcd-xhci.o
-common-obj-y += libhw.o
 
+# emulated usb devices
+common-obj-y += dev-hub.o
+common-obj-y += dev-hid.o
+common-obj-y += dev-wacom.o
+common-obj-y += dev-storage.o
+common-obj-y += dev-uas.o
+common-obj-y += dev-smartcard-reader.o
+common-obj-y += dev-audio.o
+common-obj-y += dev-serial.o
+common-obj-y += dev-network.o
+common-obj-y += dev-bluetooth.o
+
+# usb redirection
 common-obj-$(CONFIG_USB_REDIR) += redirect.o quirks.o
 
-common-obj-y += core.o combined-packet.o bus.o desc.o dev-hub.o
-common-obj-y += host-$(HOST_USB).o dev-bluetooth.o
-common-obj-y += dev-hid.o dev-storage.o dev-wacom.o
-common-obj-y += dev-serial.o dev-network.o dev-audio.o
-common-obj-y += dev-smartcard-reader.o
-common-obj-y += dev-uas.o
+# usb pass-through
+common-obj-y += host-$(HOST_USB).o