crypto: nx - Fix sparse warnings

This driver generates a large number of sparse warnings due to
two issues.

First of all the structure nx842_devdata is defined inline causing
the __rcu tag to be added to all users of it.  This easily fixed by
splitting up the struct definition.

The second issue is with kdoc markers being incomplete.  The trivial
case of nx842_exec_vas has been fixed, while the other incomplete
documentation has simply been downgraded to normal C comments.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>