docs: driver-api: usb: avoid using ReST :doc:foo markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/2cd2dc3e6bacde587aeb09a3951594cfb0102014.1623824363.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/usb/dma.rst

@@ -10,7 +10,7 @@ API overview
 
 The big picture is that USB drivers can continue to ignore most DMA issues,
 though they still must provide DMA-ready buffers (see
-:doc:`/core-api/dma-api-howto`).  That's how they've worked through
+Documentation/core-api/dma-api-howto.rst).  That's how they've worked through
 the 2.4 (and earlier) kernels, or they can now be DMA-aware.
 
 DMA-aware usb drivers:
@@ -60,7 +60,7 @@ and effects like cache-trashing can impose subtle penalties.
   force a consistent memory access ordering by using memory barriers.  It's
   not using a streaming DMA mapping, so it's good for small transfers on
   systems where the I/O would otherwise thrash an IOMMU mapping.  (See
-  :doc:`/core-api/dma-api-howto` for definitions of "coherent" and
+  Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and
   "streaming" DMA mappings.)
 
   Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
@@ -91,7 +91,7 @@ Working with existing buffers
 Existing buffers aren't usable for DMA without first being mapped into the
 DMA address space of the device.  However, most buffers passed to your
 driver can safely be used with such DMA mapping.  (See the first section
-of :doc:`/core-api/dma-api-howto`, titled "What memory is DMA-able?")
+of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")
 
 - When you're using scatterlists, you can map everything at once.  On some
   systems, this kicks in an IOMMU and turns the scatterlists into single