firmware: turris-mox-rwtm: Use container_of() instead of hwrng .priv member

firmware: turris-mox-rwtm: Use container_of() instead of hwrng .priv member

Use container_of() to get driver private structure from hwnrg structure,
instead of the hwrng's .priv member, as suggested by Herbert for another
driver.

Link: https://lore.kernel.org/soc/ZmLhQBdmg613KdET@gondor.apana.org.au/
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240831092050.23093-14-kabel@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>