subsys: logging: Fix device instance const qualifier loss

Uart device is unique and thus does not need to be passed through the
logging context. Only the assert() requires a void * cast.

On log_output side, the device usage was removed. It is actually unclear
why it has been set like this since depending on the context, it can be
anything and not specifically a device.

Fixes #27399

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>