platform: cznic: turris-omnia-mcu: fix HW_RANDOM dependency

There is still a build failure when the rwrng support is in a loadable
module but the mcu driver is built-in:

arm-linux-gnueabi-ld: drivers/platform/cznic/turris-omnia-mcu-trng.o: in function `omnia_mcu_register_trng':
turris-omnia-mcu-trng.c:(.text.omnia_mcu_register_trng+0x11c): undefined reference to `devm_hwrng_register'

Change the dependency to explicitly disallow the broken
configuration.

Fixes: 41bb142a ("platform: cznic: turris-omnia-mcu: Add support for MCU provided TRNG")
Reviewed-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20240909110417.247453-1-arnd@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>