docs: crypto: async-tx-api: fix broken code example

The code example fails to compile:

  1) addr_conv is defined twice, once as a VLA, which have been phased out

  2) submit is not a pointer, but is still dereferenced with ->

  3) The first call to async_xor() lacked the trailing semicolon

Fix these issues and while at it, fix some code style nitpicks as well:

  1) make the functions static as users are unlikely to export them

  2) include the relevant header

  3) Shorten the example a bit by removing a redundant variable
     definition

Fixes: 04ce9ab385dc ("async_xor: permit callers to pass in a 'dma/page scribble' region")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Andre Noll <maan@tuebingen.mpg.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240529-async-dma-docs-v2-1-8faf87e72e6d@pengutronix.de

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