doc: usb_device.rst: add PID for DFU in DFU mode

By default, the DFU sample uses 0x0005 as PID in the Run-Time descriptors,
and when detached to the "proper" DFU mode, it uses 0xFFFF (one of the
values suggested by the DFU 1.1 spec).

So, 0xFFFF added to the table as well as indications of Run-Time and
DFU mode on both DFU entries.

Signed-off-by: Diego Herranz <diegoherranz@diegoherranz.com>