usb: class: dfu: use the request handler provided buffer

The upload code assumes the buffer to use is the DFU provide one, and
not the one provided by the request handler, which is different in
composite mode.

This is only a theoretical issue, as this function is only executed
once the device has been re-enumerated in legacy mode, but that is a
required step before unifying legacy and composite code.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>