drivers: uart_mux: Fix compilation after logging changes

The logging macros use _Generic() atm and it started to complain
when we are trying to print pointer value of gsm_dlci and gsm_mux
structs. Cast the pointer value to (void *) to overcome this.

Fixes #35329

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>