char: hw_random: pseries-rng: Demote non-conformant kernel-doc header

char: hw_random: pseries-rng: Demote non-conformant kernel-doc header

Fixes the following W=1 kernel build warning(s):

 drivers/char/hw_random/pseries-rng.c:42: warning: Function parameter or member 'vdev' not described in 'pseries_rng_get_desired_dma'

Cc: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Lijun Pan <ljp@linux.ibm.com>
Cc: "Uwe Kleine-König" <uwe@kleine-koenig.org>
Cc: Tyrel Datwyler <tyreld@linux.ibm.com>
Cc: Michael Neuling <mikey@neuling.org>
Cc: linux-crypto@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210520121347.3467794-16-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>