docs: dmaengine: provider.rst: get rid of some warnings

docs: dmaengine: provider.rst: get rid of some warnings

Get rid of those warnings by adding extra blank lines:

    Documentation/driver-api/dmaengine/provider.rst:270: WARNING: Unexpected indentation.
    Documentation/driver-api/dmaengine/provider.rst:273: WARNING: Block quote ends without a blank line; unexpected unindent.
    Documentation/driver-api/dmaengine/provider.rst:288: WARNING: Unexpected indentation.
    Documentation/driver-api/dmaengine/provider.rst:290: WARNING: Block quote ends without a blank line; unexpected unindent.

While here, use parenthesis after get_metadata_ptr/set_metadata_len,
as, if some day someone adds a kerneldoc markup for those, it
should automatically generate a cross-reference to them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/62cf2a87b379a92c9c0e5a40c2ae8a138b01fe0a.1583250595.git.mchehab+huawei@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>