nvmem: make nvmem_bus_type constant

nvmem: make nvmem_bus_type constant

Now that the driver core can properly handle constant struct bus_type,
move the nvmem_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Srinivas Kandagatla <srini@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Srinivas Kandagatla <srini@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250712181905.6738-6-srini@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>