usb: remove a stale comment in hcd_alloc_coherent

Now that we have the local memory pool implemented there is no
need to use dma_declare_coherent_memory.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20190903084615.19161-5-hch@lst.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>